Major problem....want to stop thinking about it.......?

Ok, basically, last year I developed a friendship with this girl at school, through another friend. We pretty much became best friends, but after a little while, around the 3rd term, I started to have feelings for her, and we flirted HEAPS at school, and quite openly, which led to another friend asking me if I had feelings for this girl, which i foolishly denied (of course, lying). Anyway she got a boyfriend during the summer holidays, which left me happy for her but sort of regretting that I didnt tell her the way I felt months earlier. I didnt give it much thought throughout this year, only every now and then, until about a couple of weeks ago, during our last week of cles (it being our final year). Anyway during that last week I started to feel the way I did one year ago, and eventually admitted- just the other day- to my other friend (the one who asked me if i liked the girl) that I had started to fall for this girl......and after we had quite a lengthy discussion about it, I havent been able to stop thinking about it, as that period of time, the 3rd term to the end of year 11, would have been the happiest period of my life so far. Anyway my problem is the past few days I keep zoning out, wondering what might have been, because I keep thinking I had a genuine chance, if only I just told the girl how I felt. I am happy with the things are between us, she still a best friend, but I feel like things could have been amazing between us, something even better. Will I ever stop ruing that situation?