This kinda thing can just drive you crazy, man. I used to dwell on past decisions and wonder if I would be happier or better off if I had made a different choice, or wonder where my life might be if I had gone here or done this or said that, etc. To the point where I would wonder about stupidly small things, like, "if I hadn't called in sick to work this day, maybe I would've met someone who had changed my life," or "if I had gone out with my friends last night, who would I have met?" etc.
Eventually I just let go and decided to try and accept the course my life takes. Lead it as much as possible in the direction I want to go, but from there just let the wind take me where it may.