Infidelity Support Group
Any relationship in which one partner engages willfully in sexual relations with another outside of the partnership is considered to have experienced infidelity. This breach of trust is often traumatizing for the faithful partner as well as the relationship, and support is often needed to heal emotionally and to decide whether or not the relationship should continue after...

Cheaters lie. Cheaters don't usually change. They just go underground.
Go to a good divorce lawyer. You need a consultation. You are entitled to a lot of his money, especially since the divorce will be because of his repeated infidelity. You need to find out how much monthly alimony you will get, half his retirement account, the house, child support (do you have minor children?). I know it's a lot to think about, but the attorney will take care of everything. You will need money to live on, so don't go easy on him.
You are not powerless in all of this. Now is the time to take action, to fight for yourself.
He will never change. And you can't change another person. He is CHOOSING to do this, to cheat, to lie, to ruin his marriage and disrespect you.
Please get tested for all STDs.
Instead of feeling sad, be mad. Go get that attorney. Show your soon-to-be ex that you are not going to sit there and be his doormat.
You can do this. At least you can walk away knowing that you tried everything possible to save the marriage. None of this is about you. This is all on him, Take him to court.