Every time I get with a guy things go wrong. Everything will be fine for a while, then they either cheat or leave. Is it something I'm doing? I don't understand why I can't seem to make a relationship work. Any advice?
Either you weren't paying them enough attention, or you were too dependent. Or it could be that you didn't make any demands on them. If you never enforced any rules or had any ideas about what you wanted from the relationship, you may have come off looking like a "wet noodle."
Focus on keeping the relationship interesting. Suggest that the two of you do new activities. Find out what gestures of affection the guy appreciates and surprise him with one every once in a while.
While you're searching for a new guy, you might consider calling your exes and asking for their honest opinions about what went wrong. It might make you feel a little inferior for a while, but you're guaranteed to have a valid answer to the question you pose.
It's the type of guy's you are picking. You gotta pick guy's that are honest and into you. Its hard to find one, but you will know the type when you do.