I can't manage to let ONE day of my life go, Even a year later... Much Wisdom needed... Please share?

My graduation day was very much so ruined by the people I thought for what ever reason were my friends. My “Bestfriend” since 7th grade lied to my face and ditched me as well. I know its all the past and I defintly want to move on from it but I just can’t for the life of me stop thinking about everything that happened that day. I graduated almost a full year ago. June 9th, 2010. I drown myself in books, movies or work. Theres always going to be the time in the shower or driving my car and my brain against my will and better judgement goes right back to that day and replays it over and over and over again. I’ve left town for college, deleted my facebook and I am changing my number. I even have new friends. Why can’t I let that day leave my brain, I put worse things behind me before. I just can’t manage to let go… Please words of wisdom?