What should I do next!!?

Well.. please bare with me as I'm about to rant a little. I've never discussed this topic with anyone and this is my first post here so any help would be appreciated. Well I'm a 16 year old guy in High School and there is this girl i am completely infatuated with since my freshman year. I'm positive she liked me at one point but me, being me, I didn't flirt well or talk to her well at all and I'm pretty sure it faded. So days ped and i still liked, or crushed on her, but it was still there. In my sopre year I had my first girlfriend, a different girl, and I treated her very well.. she still liked me after we broke up. And now 6 months later im still thinking about the first girl.. when i text her she replies sometimes but ends up not responding mid convo... what does this mean? I dont really talk to her much in person (school finished, although we never talked much then)? I just really want to talk to her and get to know her and it really gets me down when she doesnt respond.:[