I banned my adult SS from my life about 2 1/2 years ago.  Best decision I ever made. In my case there is NOTHING SS could ever say or do that would make me change my mind. 

Right now you have a DH problem.  Not sure how to tell you to fix that. I do not ever bad mouth SS. I’ve told Dh that I know he loves him and that he is free to see him -  just not in our home.  All I ever said was “keep them away from me”.  They being SS and his GF who had ransacked our home when we were on vacation. 

"DH, if you want us to be one big happy family, then YOU need to get YOUR SON to treat YOUR WIFE - ME - with dignity and respect. I won't be walked all over. I will not make myself feel worse just to make you feel better. I'm disappointed that I mean so little to you that you'd expect me to be disrespected and hurt just so you can live in a fantasy that everything is okay. How is that fair to me? Why is it okay for me to be hurt? Either your son is an adult and responsible for his poor actions, or he is still a child and your poor parenting is resoonsible for his poor actions. Either way, it's not my problem. If you want it fixed, then address the poor behavior with your son. I'm not budging on this unless it's me walking out the door."

Your DH is the problem here. He should have called out his son in front of you - "Thanks, son, but you should really be thanking Tryingtomakeitwork, she's the one who made sure you could have your wedding here."

"Son, why did you ask me about getting those flowers after your SM said she was allergic?"

"Hey son, next time don't show up without a gift for your SM - she bought you and wife that X, remember?"

I'm guessing your DH has been a passive wimp all of your marriage, letting his son walk all over you. My DH would not have let his son treat me like that, but you've put up with it and he's allowed it.

Now you are changing and he doesn't like it. Well, tough shit.

I have not seen my OSD in almost 4 years. She is in her mid 40's. She doesn't feel she did anything wrong because she feels she was justified in treating the family intruder SM as a second class citizen.  

My Dh realized we were going to get a divorce with the way things were.  DH could not see the underhanded crap his DD was doing, and got defensive when I pointed it out.  But it hasn't stopped him from hoping we would all be a happy family, even though he knows I am not the one who is the barrier to that.

Your DH is unhappy because you are not allowing him to continue to live in denial with his head in the sand. You're supposed to play Mrs. Doormat to his Mr. Doormat.  Tell him to get out two sheets of paper. On one, he needs to write down all the nice things you did for SS. On the other, he needs to write down what SS has done for you.  Then question your DH why you should continue with that inequity.

Recently my DH tried to broker a "reconciliation" although OSD and I never had a relationship in the first place. I first asked DH why I would want OSD in my life after her previous behavior that she had no remorse about. Cue blank state. I then reminded DH that he can see his children on his own, they are adults so they are not on a visitation plan to daddy's house, and that healthy adults do not allow toxic people in their lives. He agrees his DD is toxic, but he says "that's just the way she is."

Fine, keep "that's the way she is" away from me!

Instead of discussing the issue inform DH that it has been discussed and you are not going to discuss it further.  Period.  If he brings up SS (NOT DSS), just say, "Hmm" and change the subject.  But do not back down from expecting DH to ensure SS respect you as his wife or SS is not welcome in your home.

I know this seems like a broken record but for 20 years I put up with the same thing from SD.  Eight years ago SD had a meltdown and something inside of me snapped.  I gave DH the option of working on the marriage or leaving - plus SD was not welcome in our home unless DH could inform her in my presence that she is to respect me in my own home.  He's never had the courage to do so.  

Like your DH, my DH is, as tog wrote, a wimp and always has been, and this has become sadly evident in other areas of our marriage.   I have found that DH lacks the back bone to support me or defend me when I really needed it and I am seriously contemplating divorce from his last wimp-out episode.

You've been married to your DH for 10 years.  You should always expect your DH to have your back, demonstrating to everyone, not only SS, that your and your marriage are his top priority.  Ask yourself if he is supportive in other ways besides issues with SS.  If you have noticed that he just hasn't been there when he should have been, you might want to ask if you really want to spend the rest of your life with a man you will eventually be losing all resect for.

If your DH sees all the nasty stuff that is done to you and still begs you to kowtow to your stepson, then really I think you are better off without him.  Roll on the divorce - but I'm sorry it has come to this.  

I would use the rest of the week then to see a good divorce lawyer, and next week go away for a week your own to get your head straight.

Do not allow him to dictate the time line, and the route the divorce takes.   Make sure YOU get the best deal you can.

I am sorry your DH has his head buried in the sand and your adult SS is so disrespectful to you. What you described your SS has done to you, after you have gone out of your way to be nice to him and his wife, sound similar to the hateful mind games my adult steps have played.

We think that once they get older, they will at least be polite. Oh no, it doesn't seem to work that way for some adult steps, including the ones I've had to deal with. Even after my DH calling them on their disrespectful behavior toward me, my DH and our marriage, the adult steps still continue to disrespect me, DH and our marriage. They are hateful to the core. 

If your DH wants to throw you into a shark tank of disrespect, and then expect you to pretend to play happy family,  I'd tell my DH he is on his own to deal with hateful DS and you want no part of it. I'd also tell my DH know that no decent man sits back and let his wife be mentally abused by a hateful adult skid. Would your DHsay nothing to an aquantince who treated you the same way?

My DH and I tried to play the happy family game with the hateful adult skids for a while, and we both hoped it would stop, until it got to a point neither of us wanted to be around them. They disrespected our marriage from day one and continue to do so. 

Don't let them get to you and concentrate on what is best for yourself.

I hope everything works out for you.

Would it be useful for you both to go to a therapist that is fluent with step families?  I only suggest it as we went to one and learned a lot about how my DH can have a relationship with the skids without involving me.  We also got a good view of healthy boundaries.  The skids are way grown and have been using guilt and manipulation (as well as their spouses) on my DH after their mother passed and then when we married.  His response was giving them tons of money so he would not lose them.  Guess what? When the $$ dried up, they were gone. And while he was giving away his retirement money to them, they were rude and disrespectful.   It took us a long time to work through this - he knows his relationship with his kids is up to him to pursue.   We have some good boundaries in place now  - as long as the boundaries are respected, I have no issues with him or the skids.  

Come on, OP, do you really want to continue living this way? Being treated like dirt, walking on eggshells, all the passive aggressiveness, your husband being a pussy to his son. I mean, FUCK.  Life is meant to be enjoyable. Things may getcricky at work or with other family members. But your home is supposed to be the sanctuary, your husband is supposed to be your rock. This is not a healthy way to live.  Wait until DSS has kids, I can guarantee you will not be welcomed at the hospital to visit, and the it’s all going to snowball from there. Get out. I say divorce him, take some time to heal, and then find someone who will cherish and respect you. You’ll be so happy you got out. 

when I am ready to divorce, dh shows remorse, backs down, even goes to our therapist by himself

So your H has a pattern of making promises that he cannot keep. He has proven that he is not a man of his word and if he manipulates you long enough you'll get over it and than the whole dysfunctional cycle begins again. This is not healthy, him gaslighting you is insulting, and I the only way he'll truly change or want to change is the day he sees you walk out that door and not look back.

The lack of respect between your H and his son is inexcusable and its sad that your H has allowed and enabled this behavior from his son. He needed to always have your back, address his son each and every time he was out of line......instead he just stepped back and watched as you were disrespected.......Your H is suppose to protect you and has done a sh!tty job at it. I hope perhaps "this time" its different but if its not that you do what's in your best interest and live your best life.....you deserve it.

Stop acting like an adult child and give this up to a higher power.  Disengage.  Don't complain to your husband about this anymore because nagging won't change anything.  Don't give SS the satisfaction of letting him know that he has succeded in hurting your feelings and nearly driving you over the edge.  Don't give him the satisfaction of running you away.  If he isn't returning calls, don't call him.  If he is lieing to you ask only 1 word response questions, like "are you okay?' and immediately remove yourself from his presence.  If you are having a family function, have your husband invite him by saying "oh, honey I didn't get around to calling SS to invite him, can you do it? I'm tied up right now."  And then get tied up.  Have your husband buy the Christmas gift by saying "honey, can you pick up SS a gift, I am all out of ideas."  If you are at the same event, try not to be near him and if you are, say hello SS, keep it moving and have a conversation with his wife or someone else in the room.  Not about him or family, but anything else...Game of Thrones...whatever.   If he comes your way, act like you don't see him.

I guarantee you that when you are no longer stalking him and he gets wind that you don't give a care, he will come groveling.

He is playing games. Time to quit playing.

Take care of you.