I don't really know what the situation is, but I just wanted to add that just because she has been with you for a long time does not mean that you have to be doing something right. When I was in high school I dated a guy for 2 years, and now I see that it was a huge mistake. I was too naive to realize what was going on.

I am not saying that your experience is the same as mine, I just wanted to point out that a long relationship doesn't equal a good relationship. YMMV