Not many of us Brits are members here - hopefully you will get more replies later when America wakes up!   Sadly, I had a very similar story to you.  I suffered abuse in childhood and it left me with a tendency to "blend into the wallpaper" when I got into a situation with 2 young step children (5 and 7 when I met DH) and their mother deliberately turned them against me. I disengaged and excluded myself from most outings etc, because younger SD just sabotaged them all if I went along - so I sacrificed myself so that the 3 of them could have a nice time.    Like you have observed - one ends up feeling rejected and lonely and spending a lot of time in your room when they are there. 

If I had my time over, I don't think I would behave like I did.  I would own my own space - I did start doing this in the later years they were coming round - and THEY stayed in their room!   I don't think that just going on as you are is an option for you - it is obviously making you unhappy.   Your SO sounds particularly insensitive in talking about his ex and sending her pics - and I would suggest that you start insisting that he takes notice of your feelings if you are to stay in the relationship.  Also - things like just appropriating your car are not on - if he wants to take his kids out he needs to get his own car.  This was one of my saving graces having my own car as at least I could go out if I wanted, on my own. 

But really, I would advise you to make it clear to your SO that there have to be changes or you're gone. Don't absent yourself in your room, you have a right to be in your own home.  Us survivors are used to making ourselves uncomfortable so that others can be comfortable - in the long run this is a terrible strategy with catastrophic effects on our mental health and happiness. 

This is an issue between you and your SO, and seems you've tried to talk to him. So time to change your actions - stop going out with them if they are going to ignore you and walk ahead of you talking about BM. Spend time out of the house if you can while they are there - not ideal with the pandemic, but if you enjoy hiking or have a friend you can spend time with socially distant, do it. Take the car, it's yours. If your SO wants you to be with them, he can include you in the group.

Seems like this will be hard for you because of your history of family abuse - a therapist might help you learn to stand up for yourself better. 

Sit your SO down and really lay it all out. The situation YOU are living in is not healthy for YOU. It's great for them! They get nice furniture, a car, stuff bought for them. Your SO gets someone tp share expenses, "warm his bed", keep him occupied when the kids aren't there, etc. If things do not drastically change, immediately, yes, I do think you should leave. 

It sounds like you are being used! You are an after "the family" thought. Do not let him gaslight you into thinking you are being "too sensative". And for the love of God, if he tries saying you are being too sensative due to the abuse you suffered as a child (trying to make his and his kids problem go away and instead your fault for being abused) leave him that very same day. 

You are worthy of far better than you are currently receiving.

Believe me my sd’s were 23 & 13 when they decided to re-enter the picture with my husband after 5.5 yrs absence. They actually tried many times walking as miniwives side by side next to my husband who was pushing our kids in the stroller.

I would tell hubby to wait a few seconds so i could put our kids baby wipes in the stroller basket or their water bottles and then proceeded walking side by side with hubby

believe me sd’s immediately walked several metres behind me and hubby because of how awkward it would have been. They tried a few more times and then i ended participating in any meets with them until daddy got them in line.

eldest sd claimed she didn’t know she did anything wrong, she actually tried telling me how to parent my kids. “Hell no biatch” in the most civilised way came out of my mouth as a long condescending NOOOOOOOOOOOOOO

I do not answer or justify my parenting to any idiot skid who lacks any basic life skills or intelligence 

He divorced, No Happy Family talk with his kids.  If they were a happy family, they will still be together.  Kids need to at least pick up after themselves.  That DH job as a parent.  
You are a new family, you will make new memories., You are number one in your family. If DH doesn't like it he can go back with BM.

Personal note.

 I just don't understand the.  we could not get along with the ex when married. To the point we could just not live together  Under the same roof as friends for the kids. That we are just so unhappy with the ex that we had to just get out.
  But we still treat the ex good.  You get it's for the kids line.  If you both cared about the kids. You should be together until the kids age out. 
But you are both selfish, wanting more out of your life, not caring about the kids.  Then you get guilty for what you did.

IMO it seems like you are being taken advantage of. You have YOUR own car, but they get to use it at their disposal like a family minivan. It's not a family car and shouldn't be treated as such when YOU aren't being treated like family. 

Don't be afraid to tell SO that the kids need to pick up after themselves and also take care of and respect your property (ex. furniture). If SO isn't understanding, I would move on. You are worth more!

P.S. - the same photo thing happened to me last summer. SD was 9 at the time and we were at a swim meet. I had taken her to practice every evening, got her swim cap and goggles ready, etc - it is getting ready to begin and then DH has the nerve to say "Let me get a pic to send your Mom" I went OFF on him! How are you expecting me to form a relationship with your SD when you are more worried about sending your ex pictures? You should be wanting to include me in any of this if you want us to be a "family." BM was aware of the date/time of the swim meet and CHOSE not to go. How is that your problem and now you feel obligated to send pics? Hasn't happened since I said something.

Youre being used. Please love yourself enough to leave this situation.

I feel you. Walking ahead. I cringe. The dads either live in a bubble and are too oblivious to even notice they are doing it. Or if they do know they are doing it (which is worse) they think it's fair because "the kids come first" or that they are making up for lost time with their darling entitled children and "you get me through the week" please! Sometimes if it's the latter, talking doesn't help anyway. They just think you are being childish and are relentless with their kids. My favourite is when they suggest a lovely family holiday and then it really gets worse. The last holiday we took I nearly got on a flight home 3 days in because everywhere we went he was walking upfront hand in hand with a kid either side, at dinner I was on the end of the table because they had to be either side, in the pool I was left on the edge for hours while he played, back in the room I'd go to bed because they were on holidays so the kids got to stay up extra late and watch movies ether side of dad until dad just fell asleep with them in the couch. Next day do it all again. One day he didn't realise he hadn't even spoken a word to me all day. At the end of the trip when he looked over with this giant smile and said "well that was a nice little trip wasn't it" I burst into tears and had a melt down. How the hell can they think we enjoy this? It's beyond me. I let it all out at the end of that trip and he said he was sorry and had no idea. But the thought of going on another trip now gives me anxiety and makes me feel sick to my stomach. My therapist validated this and said it's completely normal for us to feel this way and that going along with it would be self torture. Don't take it and stay quiet. Speak up and say how you feel. If he doesn't like it he should date his kids and not another adult woman. Also he should stop worrying and thinking about the ex at photo moments. I'd be questioning that too. Good luck. And if you're thinking about leaving anyone here will support you in doing so. This life isn't very nice.

Walking ahead. That's used to be me in the early years. We would go do something with his child they would walk ahead. I was the third wheel when he was around. Like someone suggested, I pulled back. Instead of feeling like an outsider, I went biking by myself. Went out to dinner with my friends. But then I grew apart from the child and it has had a negative impact later in life. I'd say give it another try to directly communicate how you feel. Ask him directly if he'd rather just hang with his kids alone. 

the nerve of this man to tell you are being over sensitive when you tried to open up and communicate vunerably about your childhood and things that are affecting you in the present.   A man who loves and cares for you would not have dismissed what you are saying.  Not even worth discussing to this man about how he and his kids make you feel,,, thankfully you have ended it ! good luck 

When they walked off ahead of you you should have kljust turned around, walked back to the car  and left them.  When eventually DH came whining about it you should have told him you married him and demand to be his partner and priority and if he ever loses track of that again that next time his key won't work.

I have no use for idiots like your husband.

I read your original post and it made me soooooo sad. Then read your comment and went "yipee!!!"

OK, so take a breath. Get your stuff. Document everything. Get your own place, and take care of YOU!

Good for you!  I know how difficult this is for you. Many of us do.  You did nothing wrong and this is not your fault.

Take care of you and live your new life adventure well.  Living well is both the best revenge and the best way to engage in your new life.