Can this relationship be fixed?

A few years ago I met this girl that I quickly fell for. But things never worked out for us and she changed and changed. 3 years later, I still have these feelings for her and recently we just rekindled our friendship that was broken a year ago. But this time my feelings were stronger for her and I did try to start things with her. It did not work out and she rejected me after 3 years. She isn't the girl that I known since 3 years ago. She rejected me and I told her its best to go separate ways then and just cut ties with each other. She is going out with another guy now but I regret the decision so much now. If anyone ever been through this, is this kind of stuff fixable? Was it the best thing for me to have done?