Need help telling my girlfriend I want to break up.?

I've been with her for about 6 months now but like in that time, I've started weight training and she....well...been putting on a bit too much weight. I'm not a mean person but lately she has become really clingy and all of the weight she has gained has gone straight to her neck. I love her for who she is on the inside, but me walking around school with her just makes me look bad. There is this emo chick who likes me, but I'm a senior and she's a freshman and I don't want to my gf to think I'm just shallow. I've tried to get her to go to the gym with me but she has made it clear she would rather stay at home and eat chocolate pop tarts than spend time with me, so I'm just done with her. She will seriously spend time with me anywhere but the gym. And I hate to put it this blunt, but I can't even get my arms around her when I hug her and it just sickens me. How do I break it to her that I want to break up?