Blast from the past

Yesterday I received a very interesting message on facebook. It was an absolute blast from the past.

"So, I'm supposed to be studying for my Nursing midterm but instead I decided to clean out all my old junk I have stored. I came across a box I had saved with stuff that apparently I found it important to save from grade school. On the top was a pack of pictures from a "photo shoot" we did in you room which really made me laugh. However, the next thing underneath it was a letter from you. I know this probably doesn't mean anything now but I was really hateful to you and I'm sorry. Looking back I was a bitch and for no good reason and I'm sorry. I think I was just trying to find my place and fit in but on the way there I actually lost myself. You were probably the best friend I had those years. You don't have to reply or anything I just wanted to tell you I'm sorry for the way I acted and that I wasn't a better friend to you back then."

It is the truth, she made my life hell, not only in grade school but also my freshman year of high school. I am not going to lie, I am still bitter about some of the things she did. So my question is, highschool is long over, nothing can ever be undone, should I let it all go? It probably took a lot for her to send me that message. I have moved past things, but does it warrant a response? I dont see us ever becoming close friends again, we dont live in the same city and have not run in the same circle for years. If I did respond, what the ef would I say?