Why do I keep waffling on all of my decisions?

Why can't I pick what I want to do with my life? I'm 19, graduated last year (as valedictorian) as a happy, popular kid with every thing in front of him. But almost immediately after graduating, I started to feel this impending kind of feeling in the pit of my stomach like I should do something important with my life. Problem is, I don't know what that something is. I actually attended music school (after a long and arduous journey to reach it) and ended up dropping out after about a month. "What is the purpose of my life?" keeps entering my head and I don't know what to say to silence it. In an attempt to help myself "grow up" and become more decisive, I have recently decided to join the Army.. but I keep wondering if it's the right decision, or if it's just something my subconscious has created to make me feel like I'm doing something "important"... Really, I don't know. On top of that... music school was my dream... and I left. I beat myself up every day for that one. Help..