What to do about this girl?

Lets see if I can put this in a nutshell: I really liked this girl, she was my bestfriend, I try to tell her but she finds out and doesnt want to talk about it at the time. We get out of school, summer starts, and I write a blog/book about all the girls who have influenced my life. She was included, reads it, and starts to like me back. At this point, I tried to stop liking her, because she said she didnt want to get into it at the point. I go out of town for a couple of weeks, one of our friends have a party. At the party, I hear that someone tells her Im talking to another girl(I was like whoa! It was a lie)......we fail to talk for like most of the summer, because mainly I thought she was upset with me. We began to talk more because my birthday is during the summer and we did something, turns out the entire party thing(about the other girl) was a lie. So we get to school and......well......we dont talk. Its strange because we used to talk all the time. We dont have any classes together, but that didnt stop us last year. Sooo, i was told by a CLOSE friend of ours that she doesnt like that im not talking to her.....and that if I dont start talking to her, shes gonna be really upset. I do still really like her, but im confused on what to do, because I think she flirts with two other friends of ours(who are guys) and im like really territorial, soooooo....yeah. Any advice?