Am I the problem?

My first boyfriend and I dated for two and half years. The girl he started going out with afterwards he is still with. My relationships since have been what many would consider "long term", but I'm starting to worry that its me. That I'm the problem. I'm not saying I don't recognize what I did wrong in these relationships that caused them to fail. Generally a failing relationship is a mutual thing. But what if I am THE problem. What if I am incapable of maintaining a stable relationship beyond a certain point? My dad seems incapable of maintaining relationships. He constantly complains about the people his is with. What if I'm headed down that path? What if my anxiety and insecurities eventually drive M.P. away too? =( He seems fine now, but... I don't know. I miss him.