Welcome, and good for you. Sounds like the right decision to me.  Even if you did make some mistakes (who is perfect?), he clearly didn't bring them to your attention and try to address his concerns before beginning to flirt with other women. 

And - you aren't the mother to those kids, you shouldn't be expected to act like one.  Goodbye to this Mama's Boy. 

At least be reassured of one thing - you couldn't have handled things a lot better.  You were cast in a role by your partner, his kids, MIL and his ex, and they would squeeze and cram you into that role no matter what you might have done, said or behaved. Don't take it personally - they would have done the same to any woman.  Be thankful that you are free of that toxic dynamic, and take some time for you, and to reflect on what has happened, before entering into any further relationships.  Particularly reflect on any role that you may feel you fall easily into because of how you may have been conditioned in childhood, by your parents.  This is what happened to me in my first marriage to an emotionally abusive exH, and to a degree in my current marriage where I allowed DH and his daughters to treat me poorly for a number of years.  Don't ever let someone undervalue you. 

I know you are hurting but you are free of all the dysfunction!  I'm glad for you!  What did you do wrong?  Nothing.  The only downside is we downtrodden Steptalkers might lose you. Lol.  I'm happy for you, free at last!

Good riddance to that back stabbing mama's boy, his entitled brats and his vicious co dependent weird mother.

Congratulations! You handled that perfectly. Enjoy your freedom and don't look back. 

What did you do wrong exactly? Stand up for yourself? Call him out on his sketchiness and lack of parenting? It sounds like you definitely did the right thing by leaving. It only would've gotten worse. It sounds like him and his mama were looking for someone to step in and use as a babysitter, nanny and atm when you are not their mother! That doesn't sound like much of a life for anyone. 

It took strength and courage to leave that toxic wasteland of a family. Good for you for recognizing that you couldn't thrive in that environment and for doing something about it.

Take some time for yourself. Grieve. Heal. FIll your time with things and people that bring you joy.

Then trying to make it your fault.  He has a really problem.  You were not respected by him.  Good that you left now. With out wasting more years in this relationship 

Calling you crazy and possessive is his way of minimizing and deflecting blame from what you found. Don't buy it. Being with a man with kids and a BM (or two) is really, really hard, unless he takes care to set boundaries. This one didn't. There is only so much a person can suck up and take. "Put your feelings aside." "Be the bigger person." "Control your emotional outbursts, you need therapy!" And what do these amazing dads have to sacrifice?! All for the kids my behind. Fk these failed men and Fk their babymammas. Was your life better in that relationship than before? Mine neither. I'm right there with you. 

To h@ll with the loser and his brats. There is no way you would be respected or appreciated in a situation like that. You put your time, emotions, energy and finances in this situation and get what in return? Kids who aren't even yours who do not respect you most of the time. Good riddance to that mess!

I'm sorry you're hurting right now, but you did the right thing and your life is about to get soooo much better.

We don't know what we don't know, and you didn't know what red flags to look for in a man with kids - nothing to be ashamed of. Adults learn to hide their ugly character traits, especially when they're courting/trying to find their next victim.

Keep in mind, this guy was a failure before you even met him. He failed his ex and he failed his kids by failing to keep their family intact or parent them. Then he failed you by being a sneaky, deceiving, two-faced mama's boy. He's a bad parent and a person of bad character, and you are AWESOME for booting that mess out of your life.

Good for you! You did absolutely nothing wrong, yes you will be hurt but in the long run you will feel much better about your decision. You finally have PEACE! I am in the same situation as you and all day today I've been contemplating leaving. I need to get the courage and just finally do it for my sanity. 

hell no would any inlaw tell me i was responsible for spending money on skids!! Where is their bio mum in all this?? She is somehow hypocritically excluding from being financially responsible for them!!

add to that the gaslighting, it doesn’t seem this relationship would get better

You did nothing wrong.

Don't beat yourself up over it.  I do understand the pain and grief you are working through. I had work through the same stuff during the demise of my first marriage to the serially adulterous cavern crotched whore of an XW.  Though I did nothing but what I thought a DH should do during that marriage and she was completely disconnected the whole time, I still had to grieve and hurt over the demise of that marriage.

Dedicate and focus on working through the grief and pain and .... good riddance to the the XMIL, the idiot man and the multigenerational shallow and polluted gene pool they all represent.

Enjoy your new life adventure with all of them wallowing in the stench of their genetic cesspool fading into your rear view mirror.

Take care of you. Never forget the lessons you have lived during this relationship and dedicate  yourself to not repeating those lessons.

Good luck.

Emotional adultery for him to share your marriage issues with another woman, giving intimate conversation to other women instead of you.

My stbx shares my personal info and our marital stuff with another woman too...but it is his 23 yo daughter, which is also emotional incest.

Congratulations for getting out of there. Domestic violence places have free counseling and resources...they will give you the valudation that you were abused here and that there was nothing you could do to change the enmeshed dynamic. Please be kind to you and heal!!