Problems with friends, need help?

I have an awful track record of dumping friends. Especially best friends. I have dumped 4 major best friends in the past 5 years. I don't and have never hung out with the wrong crowd, but I always end up dumping my friends. This behavior has gone for about 5 years now. Recently I made the biggest mistake of my life by dumping the very best friend I ever had, and I can't even believe I did it. I am in shock that I did it. I don't know what was going through my mind when I did it, I just remember knowing that what I was doing was stupid, but I did it anyway. I feel like the biggest jerk in the world. Does anyone have a guess as to why I do this? Like an underlying reason? The only things I can think of that could cause this are: I have trouble accepting flaws in people sometimes and I was dumped by MANY friends in the past. Some dumped me to be malicious, some dumped me for new friends, and some didnt mean to dump me but they moved away. I'm not a bad person, I just don't know why I do this. And I want to know so I can STOP this once & for all. I'm not going to continue to hurt good people in this world. I know how bad it feels to be dumped by a friend, so I don't know why I still do it. Even when in my head I'm thinking "this is wrong" Please help me!!!