I really love my girlfriend but she has so many problems with her self esteem that they are negatively affecting our relationship. She is an amazing person, yet still thinks that she is worthless. This has not only affected how open she is with me emotionally, but also how confident she is sexually. I really love her and want to stay together. But how can I help her? What can she do to help herself? How can I deal with her?
If you let people get away with putting themselves down, they will. Stop letting your girlfriend get away with it. When she says "I can't do this" or "I'm not/never going to be that" you have to get her to stop saying it. You need to remind her that she is a cool person, and you wouldn't be sticking around otherwise. Find out what's behind her insecurities and work to fix it.
The other thing is expressing to your girlfriend how much you appreciate her. I'm sure you think you already imply it, but unless you say it, it doesn't count. Her insecurity probably comes from a number of sources. You should make sure that you're not one of them. Granted, it's good to compliment her after she's just given you a mind-blowing orgasm, but that's not the only time to compliment her.
It's really about the small things. Tell her how awesome she is, how good she looks today, how impressed you are with something she accomplished. Don't tell her this in response to a negative remark she makes about herself, but tell her when she least expects it. It'll have a much greater impact and she's less able to distort these comments in her own mind, which is what she is obviously already doing to permit her self-esteem to be that low.
While you're dealing with self-esteem issues, you need to work from the physical side as well. She already has a very warped image of herself in her mind. Think fun-house mirror warped. Before you address her issues with her sexuality, you need to acknowledge that she needs to face the images in her mind.
You don't need a degree or a couch to know that unless your girlfriend feels hot in bed, she won't be. She needs to do a lot of growing on her own, but you can set an environment where she'll be able to do it.
While most college guys aren't enjoying mirrors over their beds, many need to take a good look at their rooms. Many have large breasted, air brushed women on their walls. I would suggest taking Britney off her throne above your bed while your girlfriend's over and moving your copies of Maxim out of sight (at least temporarily). Yes, I know the articles are great but really, seeing Jessica Alba on the cover won't keep your girlfriend from reaching for the light switch.
Girls shouldn't think they have to compete. These pictures send signals that those centerfold models are your ideal. No matter how secure your girlfriend is, she will look at that picture and wonder, "can I even measure up?"
After giving Britney some alone time, you need to concentrate on your girlfriend. You wrote that your sex life is following the same course as your emotional relationship. I would be surprised and interested to know if your girlfriend's ever had an orgasm -- ask. If she's busy worrying about how fat she looks or what she's doing instead of concentrating on how she's enjoying herself, there's no reason to think she's ever going to make it to the finish line.
I know girls who say they enjoy sex without reaching a climax. You can enjoy washing your hair too, and I have a good reading list she might also enjoy while she's at it; two activities that she can easily enjoy, without any final satisfaction. She's never going to love intimacy or be an active participant if she doesn't know what she's missing. Show her what she's missing.
The best way to do this is ask. Just don't go telling her she's "going to have an orgasm tonight." Maybe tell her that one night is totally about her. Don't be pushy, this will just lead her to worry more and fake it, another problem entirely. You have to show her that you care about her. If sex is too much, start with some foreplay.
Finally, you need to recognize that this is her issue. Be there and be supportive, but also limit behavior you won't accept for her and then explain why. You can help talk her into a more confident body perception. As her boyfriend, your words and actions mean a lot to her.