Hi tony.. k well, it sounds to me like she doesn't want to have sex because she is self concious. It's understandable.. i am not really overweight anymore but I used to be, so that's always in my head. And it's really hard for me to feel confident without clothes.
I don't think its necessarily your weight that she says she doesn't feel anything. It may just be that she's so focused on how she looks that she can't concentrate and feels nothing.
I definitely think that you should both start exercising together. If you are overweight enough to have it impact your life, then you should do something about it because it is probably affecting your health as well. You should take it very seriously and do this not only for her and your sex life, but your health as well.
If you are willing to exercise and go on a diet, let her know.. tell her that you would be more motivated if she came along and did it with you. It's much more fun to do it with someone anyway.
Also, make sure that you pay attention to her.. make sure that you make her feel important and not only focus on yourself during sex. Some people are very selfish and don't even realize it and it makes their partner resentful and won't want to be intimate anymore.
Hope everything works out.