I guess there was part of you hoping he would step up?
I have been through similar, what helped me was to accept that he was never going to parent the way I thought he should. He doesn't have the ability / skills / integrity.
All you can do is keep your side of the street clean, be fair, be consistent, don't put the other parent down. Speak honestly to the children ' It's just how it is for now...' Try and help them to accept it.
I think it will probably go wrong soon, and he will want to come back to you, fgs, don't get back with him.
Start to create a life you love to live, and keep being true to yourself.
I have 2 wonderful sons, 8 and 6. Their dad left me in Nov 09, and 6 weeks later they meet his new gf. They got engaged and moved in together, but split up early 2011
Then we decided to give things another go, and started dating again in August 11. We separated again in may 12 as we wanted different things. For example I wanted more children, he didn't. I want to do to uni and get a career, he thought I should just 'get a job in Tesco' in July/August he was telling me he wants me back I if I change, I ignored him.
Then in sept he meet his new gf, introduced the boys to her straight away, got engaged in October and she was pregnant by Nov. Now while I don't think the children should dictate who you do or don't have a relationship with, the boys (DS1 especially) have struggled with how fast it has all happened, and the fact that they never see him on his own anymore.
Then I found out that her children call him daddy, and that he tells people he has 4 children. I can't explain why, but this really hurts me as he doesn't have 4, he has 2. But then again, if I ever meet anyone else I'd like to know that he considered my children so important.
I don't know of that makes sense, our even why I'm posting, but I need to get the feelings off my chest.