I'd say that you are having a hard time because you keep going back to someone who's hurt you in the past. It's time to cut the strings and find someone NEW. Examine yourself and what you think you've done wrong in those relationships. Think about what it was about the other person that you didn't click with... and think about the things you did click with. Then try to find someone who has the good things and not the bad. Everyone is different but the only person you can change is you.
Was that too wordy?