I think that many of us have an amazing capacity for forgiveness. We go through so much with these addicts, and then if they seem improved, even in the slightest, we let them jump back in.
In many ways I see us as similar to women who are abused and keep letting the abuser live with them or at any rate they don't leave. I think really we are not all that much different. We let these guys that wreak havoc in our lives stay with us.
I KNOW that one reason is loneliness, and another is low self-esteem. We often believe that there are no other men out there who will be interested in us, especially if we are older, don't look like fashion models, or tied down with children. Yeah, wouldn't it be convenient and easy if the men that we already were involved with could just slip back in but this time it would work...
I read a statistic today -- of those that divorce (half of married people) 78% of women end up remarried. and that is married, who knows how many of the other 22% are living together with someone or in a relationship of some sort. I was heartened by this, basically it means that most of us are going to end up with someone else at some point in our lives.
So, my advice -- don't get yourself back into the fire with this guy, focus on yourself, if your weight is a problem then exercise or diet, I am sure you will look great! use the energy you would put into him on yourself instead.
(by the way -- I too have gained weight through my trials with an addict -- ate tons of ice cream to make myself feel better, and in the end just gained 10 pounds and made myself feel rotten about it...you never win)