hmm..

This is a toughy but let's see..

You can approach this situation in many ways actually..

But first, let me ask some questions:
1. Does she have problems with her weight?
2. Do you openly express how concerned you are about her health?
3. Is it just her weight that makes you feel unattracted to her or is it something MORE?

I think the best way for you to approach this is to honestly tell her how important it is for her to be healthy. To maintain a good diet and exercise routine. Not to lose weight but to have a healthy lifestyle. And if you can, do it with her. Consistantly give her encouragement so she knows that she's heading in the right direction. As for the foods, just eat right.. it's hard to cut out the junk food but once you substitute it with other yummy snacks, i.e. yogurt and fruits, it's actually hard to go back to the other 'crappy' food. Sometimes diets aren't for everyone.. It could take a lot time to get use to it.. but if you guys can encourage eachother, then I think that this goal is very DO-able.

It doesn't make you a bad person for wanting her to lose weight.. but I think that you shouldn't focuse on her weight. I think it's more important if you focuse on her health.. As we get older, people gain weight and it can be healthy..

You guys have been together for a really long time.. and I firmly believe that for a relationship to work, both parties has to have a very good communication line. Therefore, I believe you can tell her.. while reassuring her that you're NOT calling her FAT.. rather purely concerned for her health.

Your relationship isn't over because you don't feel attracted to her.. but it can be if your basing it on her physical looks. Because then, how can you possibly grow old with her?

::sorry if this doesn't help.. just my opinion::