How do I overcome my disgust for fat girls?

Hi, I'm an 18 year old guy. Now after reading the question your probably thinking I'm a jerk. But I'm not really a bad guy. Its just that when I see girls that are over normal size, I think *eww. she's ugly*. I admit I'm not that handsome, just average. But I'm in really good shape since I run cross country. I train regularly and take care of myself. This bad way of thinking has been with me, I think since I was 12 or 13 and I've never told anyone about it. I've kept it all in my mind. I feel bad about myself. I know they're all human beings and that no ones perfect. And it makes me sad that I can't see the beauty in them. I know a lot of them have hormonal issues and we can't really blame them for who they are. I'm still a virgin and I plan to save for marriage. I believe in one-time commitment. I'm afraid that this disorder in me might persist. Then if I marry a fat woman (or if she gets fat) I'll start feeling disgusted and all the attraction will go. How do I overcome this?