Hey, its what mumsnet is for, you can have the anonymity that you can't get in real life and say things that you would otherwise keep to yourself.
It's so totally natural to feel really upset. Right now it's very, very hard and maybe it sounds crazy but this feeling will pass. It is hard to break up any relationship, especially one where you are carrying the burden of guilt.
You sound very stuck. Whilst he's there, you can't see how you'll feel without him and he is able to try and work his way back in which leads to you being so confused. He has absolutely no reason to leave this marriage because nobody hits him, he is not on the receiving end of any of this because he is the perpetrator.
You shouldn't feel guilty because you're doing what any mother would do, that is to protect yourself and your children. Don't let him make you feel guilty because you are the one in this marriage who is doing the right thing. People who know you will know that and you don't have to give everyone details if you don't want to. There are no rules, you have to do things the way that feels right for you.
As for your children, they will be fine. Ok, maybe they will be upset at first but after a little while, they will get used to the idea and the biggest thing is that they will benefit greatly from you being happier. Children are so resilient and adaptable, I think you'll be amazed at how well they'll cope but it's so hard to see that right now.
Maybe I am wrong but I feel you need some kind of RL outlet? It feels like you are taking so much on your back with very little support. Do you think it would help if you were able to trust someone else in RL to confide in properly and to be supportive? Or does part of you feel like you don't want to burden anyone, or you feel embarrassed or too upset?