Your are right when you say,
I believe that the physical side of a relationship is as much part of it as the mental attraction.
You can love everything about a girl, but if there is no sexual attraction between the two of you then the relationship isn't going to work. People can say looks don't matter all they want, but it's not true. And no you are not being selfish, many men and women may lose attraction with their lover after X many years. It happens, you just have to figure out how you are going to handle it.
So what can you do about it? You need to find out what is putting you off of her. Is it her weight, is it the way she gets overly intoxicated, ect. When you figure it out try telling her how you are feeling. But be tasteful about it.
For example, if it's her weight you can say something like "I really want to start getting into shape and I would love if we could eat healthy and exercise together."
You can get her to change her ways without hurting her feelings.
Let my kiss steal the breath from your lips...<3