Well whenever you see her and have the chance to talk to her in private. TRUST ME!
EDIT: Apologies for double post.
Ok, here is how I take it to be, through the teenage years.
12-13 = Ok, take it slow, she is my first and I want it to be good. Month later, oh man, she didn't look pretty today, bah, never loved her anyway.
14-15 = Oh man, those boobs

. She seems nice, lets go talk, 6 months later; balls, I cheated and lost it all.
16-18 (I am taking this from friends now) = I am :censored:ing over relationships, lets party and get tonnes of you know what. Next day; "Bro, I hooked in with 5 chicks on the weekend". (I am not making this up either, it happens).
19-21 = Less focus on women, more on the future and getting a good career.
25+ = Whatver the people in here have said, being a proper man and actually knowing what love means.
So, concluding this post, it is safe to say 90% of the time a teenage relationship will end in a hole.
The truth here mate is that you never loved her. You
thought and still do think you did, what you were feeling was fake and has CLEARLY taken over your thoughts leaving you with only one state of mind. You shouldnt have made this thread seeming you cant take advice and listen to other people without arguing say you did not didnt something or other..