Emotionally unstable. Can i do about it.

I think I am too cynic and mostly think of the worst of any situation, I believe this is the reason am not able to get into a relationship. When am getting a little serious about a guy, I think I sabotage it in some way or other. This thought makes me feel like I have commitment issues. I'm just worried that the person I like will somehow hurt me, so I stop the feelings of liking him by thinking the worst. I am certainly not depressed, but I have my bad days. This has been going for years, but now I think it's high time that I change and bring in some positive attitude and love into my life and I think for that I need to change my outlook towards life that am unable to do of my own. Pls suggest what can I do about it.