Congratulations!! I know it's hard but it's going to be exhilarating to take your life back. Please be careful when you leave ...

Congrats Lilywoo! 
Your situation reminds me of mine in that it seems that the difference in parenting styles is what causes the most friction. 
Trust me, you will be better off and and a lot happier! Even if initially you're sad and confused, the peace that you will have in your new place will be worth it! Even if you wanted to keep dating until his kids grow up, but from separate homes, at least you won't be living with him to witness that whole annoying mess that is his life. 
I missed my Ss18 for like a day after he left, and I blogged about it, but overall, I miss them both a lot less than I thought. I think what I miss is not real; it's an imagined hopeful ignorant reality I hoped we could all have. I wanted us to be that family that blended well, I wanted to at least get along with them and vice versa. But lack of discipline on my DH's part and their mom PASing them demolished any chance of that ever happening. They mistrusted me, stole, lied, refused to grow up, and yes I think they knew how weak their dad was as a parent and used it all the way, even though I told them what would happen! 
Congratulations!!! You need a "I'm free" party

I"m proud of you!  

I'm sure that a big part of this is that you are mourning the relationship that "could have been".. with the "man he could have been".  The reality, if your posts were at all reflective of your life with him.. there is zero way you were happy.. and zero way that he was the right person for you.  You don't usually dredge up that much vitriol if you are just "annoyed".

Congrats on the new place.. and be safe as you transition.

The mind tricks happen every. damn. time. 
 

I am convinced you could be leaving Satan himself and you'd still have those mind tricks. Everything always looks rosier in the rearview mirror. It's less scary/unfamiliar there even if we were miserable. Unfortunately the if we stay there, we stay miserable whereas this "scary" stage doesn't last. 

Congratulations!

And please let go of the myth that it would have been better if his spawn were over 18. Sometimes, it gets worse.

The leaving is hard even when someone has used you and treated you like crap. Leaving my 2nd marriage and healing from that relationship (which was abusive on many levels) was the hardest thing I think I've ever had to get through. I KNEW he was a bad person - he really is - and I knew I would never be more than a subsidy to him. He would have destroyed me if I had stayed any longer. I loved him - he did not love me.

I am living proof you can heal. And the best way to do it is to get out, make sure you have all of your stuff and then cut him off 100%!! Staying in touch and "staying friends" is one way to seriously delay healing.

(((hugs)))

Congratulations!  Don't second guess yourself.   The exwife would never let the two of you have any peace.  Any boundaries he puts up would just be seen as a challenge to her and she'd up the game.   He would slide back to his old ways because they always want to do what's easiest.  It's a matter of time before at least one of those kids is there full time. 

Read the Adult Stepchildren forum if you already haven't.   Trust me,  it never changes. 

So he doesn't do anything you ask by way of boundaries and skid behavior. But he starts making changes when he figures out that you actually are moving out. That's about HIS comfort and not about you at all.

I know it's painful now. But pretty soon the fresh air away from him and his spawn will be sweeter for you. 

Please do not consider my pride in your courage and execution of this as condescending.  It is purely admiration and relief.

I am sorry your are hurting and grieving.

Now, stop beating yourself up over his/their crap.  Grieve, but do it separate from them, completely separately, and do it while you are out doing the things you love doing. Then when the grief ball is bouncing around in your mind and hits a feel bad wall you can immediately and willfully recall how much you just enjoyed the Wine Down Wednesday event at the symphony/opera/museum/etc.....

During one of her therapy sessions DW't therapist presented grief as a graphic where there is a square room that has a big ball bouncing around in it and when the ball hits a wall, sorrow and grief erupts. The ball is big in the begining.  Over time the ball shrinks and hits a wall less frequently.  Eventually, it is little more than an infrequent unpleasant memory in your metal room filled with great things from living well.

I came up with what I call Rags's Three Day Rule during my dating years and firmed it up during the demise of my first marriage.  A break up hurts. They all do. But, it only hurts the worst for 3 days. Then, from day 4 and each day from there going forward it hurts a bit less until eventually, it is nothing but a rare unpleasant recollection.  Unless, you re-engage at all. In which case, the pair resets to the maximum point and you have to go through it all again.

So, DO NOT RE-ENGAGE!!

Call your attorney to start the forced sale of YOUR house and let XBF deal with your lawyer.  Get your asset disposed of, your share out of it, and keep living well.

 Being over 18 doesn't alter mess at all. In actual fact I'm trying to get to grips of leaving due to various reasons and his adult children/ex wife. It won't get better. Instead you can look forwards to the atm of daddy, the weddings and all that drama and then grandkids. If you've got yourself a spineless jerk with divorce guilt and Disneyland dad syndrome, then no, it will only get worse. Don't wast time on these oxygen thieves!