Yeah, mine did that too. He still can't have them overnight so basically he's in my house every second sat and I go out. I could say f off, but the kids adore him (they're young...) and I take them to spend the weekend with my mum every so often so I get a break.
I hated, seriously hated being nice to him for the kids sake. I hated never slagging him or girlfriend (whose thankfully left the scene) off, I hated having to let him into my house, but I don't regret it one bit. Being nice to him was the best thing I did, on the advice of a friend. It has basically put me completely in the right and protected my kids.
I'm a teacher, and I see loads of kids badly affected by their parents split, but I also see kids who are not much affected by it at all. The difference? The kids who thrive have parents who are reasonably amicable - or at least, they don't slag each other off, fight all the time or put the kids in the middle.
Even my ex is beginning to be grateful about how reasonable I've been about it all (only took him three years!)
Don't get me wrong - I hate what he's done, and think he's a lying, cheating SOB, but I don't show it (yet!). The kids will work out what a d*ck he is on their own!!! And I don't let him walk over me. But I do let him get away with more than I'd like (like being in my house)
As I said, it was the hardest thing I've ever done. I remember screaming down the phone at my poor mum after he left (when the kids were in bed) because I needed to scream at someone and I couldn't scream at him.
I tell the kids the truth (they're 6 & 7) but only when they ask. I say daddy fell in love with someone else - v matter of fact. They haven't got a sense of what this really means yet, but they will, as they get older.
Anyway, sorry for the very long post, and I really know its not what you want to hear right now, but keeping the kids out of it, putting them first, encouraging them to see their dad, and being the bigger person is the best advice I ever got. The kids will not resent me when they grow up (or at least not for that), it's all on his shoulders. He can be the bad guy.
You sound like you are doing really well, in the face of such difficult circumstances. And I'm so sorry if I've overstepped the mark here - feel free to tell me to get lost!!!
Hugs xxx