Listen... you should work on yourself first so that you can become a good boyfriend.

If every relationship you have is toxic, there is one thing that's the same in every one of them: you. And, since you recognize the fact that you're a good friend but a bad boyfriend, then you need to go back and look at each relationship. Figure out which things are a pattern in them that cause the toxic nature. Then you have to figure out where those patterns are coming from.

Example: I can't stand being left out. I know this comes from my childhood of not having any friends and my cousins purposefully not including me. So, this is an area where I have trouble in my relationships when I feel left out. It's something I know is my problem, and that I have to work on. I know why I feel this way, and need to remember the person I'm with no is not connected to the situations of the past.

Therapy, self-help books, support groups, or whatever might help you. But before you get the healthy relationship you want, you have to work on making yourself the best person you can be.