Feeling alone in this life stinks.  That's why this site is so valuable!  Welcome!

If you've not read Stepmonster by Wednesday Martin, You might try that.  It helped me early on not feel so bad that I was struggling.

Hang in there!  It's not impossible, but it certainly is challenging, especially if your DH is a Disney dad. 

I feel ya!

I leave the house when I'm here without DH with SD....she sleeps late so I try to be gone before she wakes up. 

Time to FULLY disengage and refuse to look after sd when dh is work.

he doesn't get to dismiss your feelings, refuse to support your right to basic manners from sd and then dump sd and friend in your lap while effectively muzzling you through fear e.g. your tip toeing around them is demonstrative of this.

• SD was 7 when we met and everything seemed great in the beginning.

It does doesn't it?  Fun!  Everyone is new to each other.

• After we had been together for a while I started realizing DH is a Disney dad. SD didn’t know what it meant to be told no which then turned into SD telling DH I was mean because I expected basic manners and respect. DH would also immediately get defensive if I said anything to SD.

Been there, done that.  DH still is protective of both SDs and is super sensitive to anything I say.  Agree with the comment about read Stepmonster.  This is what drove me to StepTalk - the bind of responsibility without authority.

• This hurt me a lot because SD and I would do all sorts of fun things together.

It's always better when the skid is having fun!  You are the fun one!  You are not a parent!  You are there to entertain and be back up 'mommy' but only the fun parts and pay for things!  But...slippery slope - if you start NOT doing such fun things then the games begin and now you are selfish and mean.

• At that point I realized I needed to disengage. I’ve been disengaged ever since but it seems SD gets more disrespectful as time goes on and she gets older. DH leaves SD with me when he has to work and SD will hardly say a word to me and spends all of her time on her phone.

Why does your DH leave her with you?  Are you not working?  Does he not have access to a child-minder or child care other than you?  Have you and he discussed, like, really seriously sat down and discussed how if you are RESPONSIBLE for her under your care that you have the AUTHORITY to make your adult decisions?  If he doesn't agree then he gets to hire a child minder or whatever - and does he give that child minder the AUTHORITY over his daughter? 

Second - if she ignores you, good. Let her be. If you have no authority to get her off the phone and make her interact, then good for you.  Ex:  I've been alone with SD14 all day. I've seen her once for a few quick words, but other than that she's been in her room.  I've been up and doing things for 9 hours.  She's been in her room for 8.5 of those hours. Not.my.problem. She feeds herself, DH cleans it up. 

• She doesn’t pick up after herself and expects to be catered to and always get her way.

Then you don't pick up after her, right?  You are disengaged?  DH picks it up or it gets trashed, or donated or "disappears" right?  I had this issue.  DH either cleaned it up or after 24 hours it was clear no one else cared, so I did with it what I wanted.  And get her way with what?  Example? OSD would always want some horrible crap meal but that does't mean I cooked it.  It meant I cooked a good meal for everyone and if she didn't want it then she and/or DH could make something else AFTER I cooked our dinner. If getting her way meant something I disagreed with, I either ignored, or refused to participate in. No wishy-washiness.

• SD also doesn’t know how to entertain herself or be independent which means I am normally left with her and at least one of her friends.

OK, and if she has a friend over, do they entertain themselves?  If an 11 yo (SD11, right?), then they can be clingy. I more than once told either SD that "that was enough I need to do things and go find something else to do otherwise I WILL give you something to do..." i.e. vacuuming, dusting, cleaning the kitchen, weeding the front yard, etc.  It only took a couple times of me saying this before they stopped.

• I feel like a prisoner in my own home when SD is here because I have to make sure I don't say or do anything to upset her. I don’t know how to talk to DH about this because he gets so defensive.

If you are really disengaged then there should be no reason or need to say anything.  She can make her own food right?  (I'm betting this is part of the problem) If not, show her how to make 2 basic things, let your DH know and be done with it. Don't address her except for being polite.  Yes, No thank you, pass the beans please, Good night, Good morning - what else is there? 

I spoke with my DH about this in short bursts. Like something happens, let it rest for a day, then on neutral time, with no SD there, talk about it with him.  Use "I" terms not "you" or "she" terms, keep it neutral.  Do not get angry or upset. "Hey, DH, I was feeling X the other day when I was 'cut out of the conversation/was told to not say xyz to SD/expected to cook lunch for her' - I was wondering if there's a reason why you feel that it's important that I do this.  And if so, since you have given me the responsibility for doing xyz, it would be helpful to me to have authority to decide on xyz."  Then give an example of how you see yourself doing it differently (do NOT bring up what actually happened! Do not tattle on the SD!) and discuss. 

I met my DH when his daughter was 8 or 9. Now she is in her twenties and things are not better. 

The thing about desengaging is sometimes it backfires. Or it did for me because sometimes ignoring the SD attitude or not saying anything when she leaves a mess and let DH pick it up, is SD takes that as your scared of her or you are not allowed to say anything to her. I hope I am explaining correctly.

Anyway good luck because over the years of reading these blogs and being a stepmom for over 10 years I can say it does not get better unless your DH handles her.

You give your DH two choices. When he is not home, and SD is there, you have full authority to parent her in any way you see fit and he will back you up 100 percent. If he doesn't want to give you that authority, then he hires a babysitter or sends her to daycare when he is not there.

I'm getting a strong vibe that you are afraid of your dh's anger. Does he yell/scream/throw shit? Or are you just that conflict avoidant? If his anger scares you, end it. There isn't anything to save here.