Experiencing a decline in my feelings for my girlfriend.

I'm a 19-year-old guy in a relationship with my 19-year-old girlfriend. We've been together for just over two years, having met in high school. I genuinely care about her, and our families have been supportive of our relationship, helping each other with various life challenges. I’ve always been active and compete in martial arts, so I stay in good shape. When I first met her, I found her incredibly attractive. However, since starting university, she has gained some weight and feels self-conscious about her body. It’s reached a point where she prefers to wear a shirt during intimate moments, and I haven't seen her waist or hips in over a year, which I find particularly attractive in women. I understand that my feelings about her body are my problem, and I recognize that I haven't been very body positive. Despite my efforts to remain attracted to her, I find myself drawn to other fit girls who share my interests. While my girlfriend has started going to the gym, she hasn't made significant changes, and her appearance hasn't improved much. I'm unsure how to approach this topic with her, as I'm worried that if I mention her weight in a negative light, it might lead to a confrontation. I acknowledge that I'm not handling this situation well, and I understand any frustrations with my perspective. I'm looking for advice on how to discuss this sensitively without hurting her feelings.