Why does it hurt so much to see him with her?

Now I know I'm gonna sound ridiculous, but every time I see my former crush (I had a HUGE crush on him) with my "friend" (We were great friends at the time I told her I really, really, really liked him) it stings, or just hurts a lot and I feel miserable with my life at the current moment - because I feel their love growing. I can sense it from a mile away. I haven't fully accepted their relationship, maybe I have.. but why did I spend days wondering why I wasn't the chosen one or why I'm never the lucky girl - the girl who got the guy. I still can't take them being together, because it hurts me too much. It has gotten so bad that I even got to the point where I try to avoid seeing them on facebook chat whenever either of them is on. What can I do to stop worrying about them, and meet a guy who will make me the lucky girl?