Many of my friends have had similar, but if you can make it through the first few years of baby/toddler, then most people seem to stay together.
It's tough, it''s not fair, and I get that it's hard on you. I didn't make it with my first relationship, we broke up when our child was 1. He just made my life harder.
The only thing that seemed to help, if it's any use at all, is to a} make the absolute most of enjoying your children, make sure you aren't just doing the drudgery, leave some mess for him to clear up/don't cook for him/order in take always just for you if he's away on a long stint, join lots of good toddler groups.
B} get loads of support, a good friend network. When he does appear, ignore him and go out, with friends, even on your own to the theatre, anywhere.
C} Whenever possible, give him sole charge of the kids for as long as you can stand, and go off out. Even if they make a great big mess, it can be the only thing that opens his eyes to what you do, and the only thing to promote a bond between them.