Venting here, and reading here as well as commenting here, that should help release some of the steam that you have building up.

Its important to realize that you are feeling resentful because you have taken on someone elses kiddos. There is no way you could ever truly "know what you signed up for", as the world likes to tell us.

I too, stepped up to the plate and I am also resentful of the lazy Toxic Troll. I just doesnt seem fair that they are golden And dont do much except exist and we get to pick up the extensive slack.

Try expressing your feelings in a less emotional context. Try seeking a good therapist to help with dealing with the feelings.

I have no advice because I am just as angry at my own sister for what kind of mother she is to my niece, who I love and who depends more on my DH and I for things and emotional support, than her own mother. She wants that support from her mother, which I understand.

My sister is selfish (and an alcoholic) and DH and I have made a lot of sacrifices over the last 18 years for my niece. And, I have never wanted children, but there you go, I have one in my niece!

So, I am angry about how my sister has made my niece feel about herself (unworthy of love), but there is also resentment for what I have had to go through, and am still going through, for my niece (because of my sister). I never got to be just the fun aunt.

Your partner should be picking up any slack for his own daughter, should not expect you to do extra work, and really has no right to be angry at you because BM does not do her share. What does your husband do for his children?

What is the 18 year old's plan? College? Job? Moving out date?

My 3 SKs moved in here, one after the other during a 9- month period.   They were 15, 13 and 9.  I already had DS12 and DD 10 here.  BM's only contribution thereafter was her critique of our parenting.

The thing that bugged me most was how devoted the kids were to her.  I never received a scrap of appreciation from them or DH for raising them.  I didnt speak harshy of her but they could probably tell how I felt.  We never had one kid-free weekend after they arrived.  My ex was just as bad and we had no grandparent support. BM seemed focused on her new boyfriend and all the drama she stirred about around him.  They vacationed at a nearby lake every weekend and never took one of the kids altho they begged to go.

Going to counseling helped me a lot and so did going back to work.  I feel for you, it's not fair.  Sigh....

Get the skids out of the house for one. I know it's not always feasible at 18, but if you amd DH do it right, you won't have many more years of living with them. 

Not the same thing at all, but I want to share a situation about something from the past that can't be undone. My DH is furious about a guy I dated before I dated DH. How could I date that trash, etc. etc. Yeah, he was a boy toy and a divorce rebound but it was sure fun while it lasted, and the bright flame burned out quickly. DH was, and is, absolutely irrational about it, he knew it, yet the same crap kept coming out of his mouth every time he felt insecure or we were arguing about something else.

And it hurt me whenever DH brought it up. There was nothing I could do about it. I asked DH what he needed from me so that he could let go of his anger. He wanted me to apologize. Hell to the no, I wasn't even dating DH at the time and I wasn't going to make an insincere apology for something I wasn't sorry for. I enjoyed that irresponsible time with Boy Toy, and DH just hates that. (But, oh, don't ask about HIS dalliances because that's different somehow.)

Eventually I couldn't take it anymore. I told DH he needed to work this out on his own, stop punishing me for his own insecurity because I wasn't going to listen to it anymore. And DH did. He must have discussed it with his therapist and worked to change what had become a bad, hurtful habit. He no longer mentions Boy Toy.

So, all that to ask you -- is there anything anybody can DO about the situation? Is your DH handling his parental responsibilities? Is the SS contributing to the household or at least not bringing more drama and work to your doorstep? And what does your DH do for YOU to reduce your stress?

Individual counseling for you might help. It will give you a safe place to blow off steam and work on strategies for coping with a situation that is unfixable.

I totally get your resentment,  Mine does not come out daily but it does come out.  A lot of times I will say things but it's not a ton.  Usually I vent to ppl that I know, etc.  Basilcally, my BM is nuts.. I am not just saying that.  She gave us my SD who was 6 at the time ONE week after we got married.  She has basically done everything to break us up under the sun.  Suicide threats, telling me she loves him, CPS called multiple times, cops called on me- oh maintains a relationship with DH's family.  Anyways, I feel the resentment as well.  It has lessened over time because I really do a lot less than I used to.  Now I think what kind of person wants to miss out on a child's life just to try and "ruin" someone else.  I am sure the BM figured I wouldn't stick around once we had the kid here.  To be honest I almost left.  It was within 6 weeks I was looking up divorces.  But I have stuck it out so far.  Hopefully some day it's worth it.  

Go talk to someone- write your feelings down and understand what you feel is COMPLETELY normal.  I actually feel your pain so much.  My house used to sparkle and now I don't clean as much... I don't do a ton for my SD only because the mother is so toxic and I want really nothing to do w/ it.  But I get the resentment.  Maybe feel pity toward someone that is so wrapped up in themselves and cannot get over the past.  
 

You are doing more then the birth parent.  Having your life control by other parent because they don't do anything for there kids.  This is what no one knows about stephell.  You have a right to have Resentment!!  Unfortunately your SO will never see that.  
You must do what you have to do, it's your life, you only live once.

We ALL totally get it. Few things hurt as much as some outsider messing with your family and your life like a game. Or just dumping their responsibilities on you......
The anger and resentment are real and deep. 
However, those negative feelings can be destructive forces even when their justified. A counselor could help. Journaling has been helpful for me. Try blogging and venting on here. It's safe and anonymous. If you have trusted friend you can confide that helps too. Carrying around the anger will hurt you, and venting it every day can hurt your family. I think you said your raising your own child too? So you want your little one in a positive environment if possible. 
It's not realistic to never say a negative thing but you can find ways to cope with negative feelings so they don't run wild. Bottling it up won't work. You need a safe outlet for your own mental health 

hugs

Make a different choice on how you feel.

Feelings are a choice. Whether someone wants to recogize that fact or not.