I lost touch with my best friend a while ago, why is it bugging me now?

so in gr.5 i met my best friend, we had a fallout in grade 6 because of different classes but grade 7,8 and 9 we were like attached at the hip. She was the person that i could truly be myself around, i don't remember one time where we hung out and didn't spend the whole time laughing at absolutely nothing, people always thought we were high but we just had so much fun together. Anyways, in grade 9 i got my first serious boyfriend so we didn't see each other as much as we used too but we were still close, typical high school changes happened like making other friends, and she started hanging out with a group that was really into drugs. I didn't want to be around that so it made it even harder for us to hangout and i guess she felt like i was too busy for her with my boyfriend and by grade 10 we barely even talked anymore. There was once where we were both on msn a few years ago and i said that we should hangout sometime and i missed her and she was like ya i don't even know what happened, but we never ended up actually hanging out. Now i'm going into second year university and everyone is moving on and forward with their lives but all of a sudden i feel like i am stuck in the past and am so upset over the loss of this friendship but i don't know why this is happening all of a sudden, this is years later how come i am feeling so torn about it now?