How do you know when it’s time to continue holding on or time to let go?

About three years ago I met my best friend, Nick. We were in seventh grade, and he’d just come to live with his father.
After a few months we started dating. But that didn’t last long. I thought he deserved better. I was falling in love with him. But I wanted him to have someone better than me.
So I broke up with him.
We didn’t talk for a while, but then we had a class together, and we were best friends again, instantly.
I never really got over him.
I went to homeschool the next year, because my parents were getting a divorce, and my mother and I were moving a lot.
He moved five thousand miles away durring that time…
Last night I was talking on the phone with him. And he asked how long I liked him after we broke up. I decided to stick with the truth, and tell him “I never stopped.”
He asked if I still loved him. I said “Yeah… I do.” and he said “I’ve been in love with you since the seventh grade. Probably always will be. But AshLeigh, you gotta move on. I’m no good for you.”
We talked for six hours. I think I cried for four of them… I think we talked about everything. Everything I wanted to know. And for the most part I liked his answers. Except the one that said he’s still not coming back.

I went through everything with that kid. I thought I was getting over it. But now… I just don’t know anymore.
It took so long just to feel alright. I thought I’d be okay. But I’m not anymore.
What do I do? How do you know when to move on…