Is this friendship going to work.. need advice?

Well my best friend is Lexie and her and I have been friends since the 8th grade, I was crying on the bus because I found out my first kiss cheated on me.. it was right before summer started.. so that summer we hung out and we just clicked. But the past 6 months our friendship has been going sour. For example, she randomly gets pissed at me over things that are not a big deal. Second, I have caught her multiple times not telling me something important that she should've told me.. like a guy she liked or that she is having hip problems and needs to go to the doctor.. she doesn't tell me anything important anymore but she will tell everyone else. At school she acts like my best friend but then when I go over to spend the night she gets in a crappy mood. We used to play music together, she would play guitar and sing backup vocals and then I would sing main mostly and it worked out fine, but then she started critiquing me and I would try to change but it didn't seem to satisfy her or something, I've offered her to sing main or play a song so then she starts playing, and when I come in with backup she stops playing. She is just acting like a complete jerk and I am getting sick of her attitude. She is supposed to be my best friend ever but I just completely forget why I am friends with her in the first place. I don't feel close to her anymore. What should I do?