First of all, I would like to congratulate you for deciding to be done with men unworthy of your attention and time.
welcome to the board. i'm proud of you for deciding to date someone worthy of you. sometimes self-sabotaging behavior stems from fear of success. good, strong relationships take a lot of work, and most of the time that can seem overwhelming. so your gut may be telling you that going out with nice guys is too hard and that you should just give up before you start. it would be easier to go out with not-so-nice guys, since you've done that before, right? so, since you want something new, this is going to be a challenge for you, but you can do it.
sometimes it helps to write your beliefs down so that you can see them in front of you, so maybe you can try making a list. three columns: first header, "new belief" (such as "i deserve love"); second header, "old belief" (such as "i don't deserve love"); third header, "discrepancies" (write down the difference between the first and second ones if they don't agree). maybe this will give you some insight as to why you act differently than you want to. again, welcome to the board and we look forward to hearing more about how you're doing.