It's obvious you truly love this man. It's my opinion the most loving thing you could do for him would be to start a divorce. You're not helping either of you by allowing this to continue without taking action. Sadly, I can't think of any other action besides divorce. If he has a change of heart, you can always give him another chance.
It's easy to think very little of yourself having to endure what you're going through, but maybe this will help... There's not a lot of women out there who would stand by a man's side through as much pain as he's dealt onto you. In my opinion, you're a champion.
I hope the best for you and your husband, and also, hope that you can hang onto the level of goodness you've shown yourself capable of.