In the vein of lovefromgirl's story - you do have kids to think about. I've read a few studies that have shown that kids are more likely to be well adjusted with divorced but amicable parents than married parents who constantly fight. Just something to think about if it comes down to it. If you two split, it doesn't mean your kids don't have a dad, it means you don't have a partner.
Personally, I could never trust this man, and I think that's what it boils down to. Will you EVER be able to FULLY trust him again? Even if he does the work, grows up, and starts acting like a responsible adult consistently - are YOU going to be able to forget the past transgressions? Wanting it to work isn't enough. Forgiveness from you will be key.