I know that I have a problem just don't know what it is or how to stop it

Okay here it goes, I'm 42 and single I can't seem to keep a relationship for more than a few months, the only common thing is me. I find myself threw the years to occasionaly find a woman that makes me want to be in a relationship and they dump me, I never have a hard time getting dates but it seems that I go on a few or date for a wile and I loose interest in the others. I have no idea what to do, I either just date them for a short time and find myself questioning things and I then end it. then there are the women I have no question about and they allway end it. The last one I fell quick and hard she allways wanted me around then out of no were blew me off. I know there is something I am doing but I have no Idea. I have sunk into a funk that I can't describe and I keep going over all my past relationships not just this last one. I don't know what to do anyone have some advise suggestions anything......