How can I stop my mood swings and my being emotional?

Sometimes just a small thing can mess up my whole day, and I feel like crap. Yesterday it started like that, then I received my provisional driving licence later that morning and I was back up again. What can I do to control my mood? Plus, I feel like I'm too emotional sometimes, too hard to let go. A best friend of mine left me (betrayal) a month ago, and she's posting some bad posts on her blog, often about me, and that makes me really feel bad, so I try to hide everything by pretending I don't care, even though I know deep inside that I do care, because friends are hard to leave, especially best friends. However, I think I will explode one day if I keep on taking everything in... Is it teenage years? Is there something I can do to get rid of all this emotional rollercoaster?