He definitely picked the wrong person.?

There was this one guy that I have really fallen for, I found out that he is dating someone else the other day, and I think that was a huge mistake. I for some reason, have a feeling that I am really perfect for him, I mean we have so so so much in common unlike the other girl, I am his perfect match and he is totally oblivious to it. I have never felt this way about someone and it feels so heartbreaking for this to happen. My friends even tell me that he has made a mistake, I just cannot believe what has happened. I told him a while ago before he started dating that girl that I liked him and he didn't say anything just that he is glad. Now I just can't stand this he has no idea about how hurt I am about this and is getting mad that I will not talk to him because I just can't. Should I just let it go, or what? How can I show him that I AM the one for him not the other girl.