Teenager years are a mess...
We go through a lot of situations we can't think of and at the end all we are left with are the memories... whether good or bad but the thing is after everything we learn to live anyway. There are moments which made us happy at some point of time and those are the reasons for the sadness in present.
When we can't have the person we want to be with, we start to believe that it's our fault it could have been better if we had done this or that way or said this or that and day by day it leads to increase in stress and anxiety. Somewhere in our heart we still hope they'll come back, we'll be together once again just because the poor heart of us can't bear the pain of separation.