Any suggestions on what to do? Please Help!!!!?

There is this girl I've known for 2 and half years and I don't know why, but she doesn't talk to me that much. You use to talk to me a lot, but she has been acting wierd. I'm a intelligent and not a y guy. We're both in 9th grade. The problem is that I'm a shy guy around her and I don't have a high amount of confidence. Also, I have sometimes very low self-esteem. I think she might like another guy and I have been getting heartaches on what to do. I feel like I should either quit our friendship or ask her out. I don't know what to do? When we're both in the on cl that I have with her, she seems to stare at me. I can on the tip of my eye, I can see her staring at me or something in my direction since we sit on the opposite side of the room. She has looked very depressed, sad, or something lately in english. I don't know why? What should I do? Please Help! Thank you for people who respond to this question. Finally, please no stupid or dumb replys to this question.