[Jaguar]

I'm sympathetic. I had a girlfriend like that, too. It usually comes from one or both of two places. She may be from such a strict and sex-phobic upbringing that she is responding to that by not letting you see her naked, or she may have a very negative self image about her body. Even very attractive girls will tell you they're overweight when they're not, or their legs are too short, or their boobs are too small or too large (they never seem to be just right).

The fact that she's having sex with you sort of precludes the first, so I'll assume the second. It's going to be hard to convince her that she's attractive. Maybe she'll let you light a candle after you have sex and you can tell her that she's perfectly proportioned, that she looks like a painting in a museum, that she has a figure like a statue, etc. You get the idea. Use your imagination but don't overdo it and use all your lines the first time. Words mean an awful lot to girls, especially good words.
She may never want to go skinny dipping in a group but I think some confidence is what she needs.