This will sound a bit harsh, so I apologize in advance.

You are both kids. This relationship was doomed from the start since most people do not end up marrying their childhood sweetheart. You are way, WAY too young to be thinking about a girl in a marital way regardless of how much "experience" you believe you have. Males are generally not ready for marriage until they are pushing 30, and even then, lots of them aren't ready.

Your college buddies are right to some extent. While I don't condone flagrant promiscuity, I do think you ought to be focusing more on your personal development and less on such intensity in a realtionship. It looks to me like you are trying to force a fit where one doesn't necessarily exist.