I was in a very similar situation. My ex was always negative about anything I wanted to do.
Have you tried counseling yet? I would certainly suggest it as a first shot. I went with my ex, and eventually when he decided to quit, I went on my own. I also went to my doctor and got a prescription for antidepressants. While neither saved my marriage (it takes two to tango), it did help me to deal with everything, and make the decision to leave.
I still go to counseling one a month, and it really helps me to deal with my ex and moving on with my life.
You can often find free or low cost counseling through a church or your local hospital. If you're in the South Bay (LA) area, I can give you the info for the place I go.
Whatever you decide to do (go or stay), be sure to surround yourself with supportive friends and family. They can often be just the thing to keep you going when you can't deal with things on your own.