First, don't be depressed. If your girlfriend's family is anything like the "in-laws" the rest of us deal with, it's probably a good thing they don't like you.
Second, only focus on what you can control: be cordial, helpful, and polite. Don't give them a legitimate reason to dislike you (e.g. going off and smacking an annoying, yapping five-year-old nephew).
Third, honor, love, and care for your girlfriend — make her happy. In the end, that's the best her family could ever hope for.