[Jess~]

I think the other replies have covered most of what I would say to you anyway, so I just want to add that a guy losing his erection isn't always because he's not attracted to what he's seeing. I know it makes sense... erection = turned on, so wouldn't a loss of one = not turned on?
However, I've read quite a few articles about this issue that beg to differ. There are SO many reasons for a guy losing his erection. I highly recommend you do some research and see if any of these other reasons might apply to your situation.

Also, try not to be so hard on yourself. He's dating you, so he has to like something about you. Talk to him. No matter how the conversation with your boyfriend about this, if you choose to do so, goes, the only person's opinion of you that should matter is YOUR opinion of you. So if you don't like yourself, work on yourself. People always say embrace yourself and learn to love yourself, but as a workout junkie I just have to say no. If you don't like something about your body, you CAN usually change it. So work towards that goal, whilst simultaneously working on accepting yourself more, and don't give up. (By the way, thick legs aren't a bad thing. Not that it matters as much as your self-love matters, but a lot of guys love a thicker, curvier lower body. Embrace the good things about yourself and work on the things you perceive as bad.)