M.D.
You can't look at marriage like that. There is no such thing as 50/50, you both have to give it 100% all the time for there to be anything worth saving, fighting for, keeping. Maybe he DOES want to change, but alcoholism is a real problem. Why hasn't he sought, or you demanded, he get help for that? I don't think either my husband or myself would deal with the other one being an addict to anything without seeking professional help.
I suggest you go to counseling. If not to give you the strength to leave him, to give you the strength to know you are better than he makes you feel and you deserve better. Don't stop going. You need self-esteem and then you'll be able to make a clear decision.