First take control of your income. Have an honest talk with him about money. Every single one of us has faults. Mr. Wonderful-that-makes-you-laugh has baggage too, you just don't see it yet. PLEASE PLEASE PLEASE take care of your marriage one way or another before even thinking about someone else. This has such dire consequences. You want so much to escape the situation you are in that you over look the fire you're jumping into.

Take your power back. Close your joint account, open one of your own, close the joint credit cards etc. Good luck!