However, strange as it may seem, I was and still am in a similar situation. Girl I knew since 8th grade. I could never bring myself to say anything. Graduation came, and I didn't say anything. Saw her a couple times in college...still couldn't bring myself to do anything.
To this day, I still think about what I want to tell her, but I still don't know if I can break my silence. I wish to badly that I could see her before the 10 year reunion for HS graduation, assuming there even is one, but at the rate I'm going, I don't see that happening.
Best Case Scenario: She does like you, and you need to just say something, no matter how awkward it may come out. If it helps, mentally rehearse it. Have everything you're going to say worked out long before you pick the moment to say anything.
If you know of some common ground where you can start your conversation at, that may also help you feel more comfortable.
Worst Case Scenario: She feels sorry for you (for some reason) and she's only being nice because of it. There's practically no chance of this, but one never knows.