I am sorry I do not know anything about the mental issues you have.
I am sorry your marriage is not happy at the time.
It would be nice if your wife had a higher self esteem, maybe then she would not feel she needed support from you. But either way it would be nice if you did give her more support. If she tells you she feels you don't love her and she thinks all you care about is being with a woman with a perfect body etc, then that must be how she feels!!!!
You may think she is wrong....but she feels that way for a reason!
No matter her size woman want to know they are attractive to their partner. Maybe if you showed her that she is attractive to you she would take better care of herself! Build her confidence and then hopefully she can take over from there.
If it would help fantasize about someone else if it would help you get ready for sex with your wife. I did not say have sex with anyone but your wife!!!!
Good Luck