You said it. Leave while you can! 

There is a nice man out there for you, with none of this baggage. 

Will not get better. In fact, it will get worse. 

Run. Like your ass is on fire and a monster riding a pack of rabid wolves is chasing you. 

Hi, I'm sure its hard to hear "leave while you can" and that really comes down to how you feel about your BFand his efforts in helping you in feeling supported in your house. 

That said it sounds really hard, and only you know if you can disengage from the kids and still be happy. This might require some SERIOUS negotiation on BF's part, and having your own space that is always child free. Making sure rules are obeyed and behavioural management plans are clear and its all coming frmo him to the kids. Please and thank you's are annoying, but if my SD doesn't say please, I say 'no, because you didn't say please'. If I give her something and she doesn't say thanks, I take it back immediately. It feels so damn mean! but she learnt pretty quickly. 

The fact that you said "we" are trying to encourage some basic chores and routines is encouraging. You're working as a team at the moment. I know its only been 18 months, but I wish my DP and I had started relationship psychology earlier to help us. It's been pretty good (probably due to go back now though haha!). Remember that they're his kids, and try to make a little time to have fun with them. You definately  need your own space though!

Regarding BM - there's not much you can do. All I can suggest that's working with SD's BM and my son's step mother is small gestures over time. I will send my son with flowers for his step mum. I'll go out of my way to get DP to ask BM how she would like 'X' situation dealt with so she doesn't feel stepped on, but we've been at it longer than you. She's probably still hurt he's moved on, so you might need to give that one time. 

4 kids. All out of control and not parented. A manipulative 8 yr old who will turn into a teenage nightmare.  And a difficult BM.  You are young and no kids.  Like I was when I got together w my DH. 3 kids - a teenager girl nightmare. If someone told me how awful it would be I wouldn't have believed it. But honestly it's so much worse.  It does not get better as they get older, it gets harder.  You do not want this life. Go enjoy life without all this baggage.

Do you live with BF in his house?  If not, and you have your own place, don't give it up.  Don't move in and whatever you do, don't marry him.  I think if you are not sure what to do, 6 months down the line, you probably will be. 

Do I leave while I can?

Yes, please.

Honestly, I don't see how you can stay. Why would you put yourself through that?

I am 44. I met my ex in my early thirties. I had no wrinkles. I now have under eye wrinkles....

I regret not getting out sooner. I would probably look a bit more bright eyed and bushy tailed. 

Please don’t spend your entire thirties on a situation  you are not sure about. If you become sure about it that’s a different thing entirely. Please don’t wake up on your 39th/40th Birthday full of regrets. 

These should be the best years of your life. 

Ok, I am going to be the odd man out and ask more questions before I say leave...LOL

1. What is your relationship like when you don't have the kids there?
2. Do you live together?
3. What is the actual custody arrangement? Do they have a solid CO? 
4. Does your BF back you up when the kids are acting like fools?
5. What does he do when the youngest is mean? 

And you are still voluntarily sacrificing yourself to this alter of Sparental martyrdom, failed parenting and unintelligent breeding partner choices why?