Morbid obesity and health issues causing my DB

My partner is over 500 lbs and can barely move day to day. She has a terrible back, horrible knees and no energy so sex is the last thing on her mind. We haven’t had sex in a while now and when we used to have sex, it was the same position every time and me doing all the work. Now I miss even that because it is completely dead now. I don’t know what to do or how to help her lose weight. That goes beyond sex but her overall well being in general. The rejection and hurt is so deep now, I don’t know what to do.