kids angle
It sounds like a lot of the reason you still love her so much is you are a good family man, you love your kids and you know she loves them a lot too and is a good mother. Given that have you tried discussing the implications of her weight on your kids with her?
My mother is 58, is an absolutely amazing woman, but has been 250ish for 30 years and is on a suitcase full of pills a day for health issues. The biggest issue in our relationship has always been some form of animosity. I have felt shorted by the fact that she chooses food and laziness over her own health and over me.
I always thought that even if it wasn't enough to do it for herself she would want to do it for me, that she would want to see me get married, see her grandchildren, etc... The reality is she may never see these things for me or my brothers and sisters and its a direct result of things she has CHOSEN to do to herself. Not cancer, or some other disease, but a choice. Its no different than choosing a drug habit that ruins your health and makes you die young over your husband and children...
I know its really hard, it was very emotional when i verbalized these feelings with my mom... But please do. I sure wish my dad would have at least tried when my mom was 31, maybe if this issue had been addressed earlier somethign could have been done....
p.s. my mom and dad remained married, going on 37 years, but I'll tell ya what, none of us kids have a positive relationship with him b/c he is an angry/ bitter man... I cant help but think some of his issues stem from the exact type of relationship you are describing...