I am 19 years old, and for a long time I have had a major problem with my thought process. For a while I just thought it was normal, but I have come to believe it must be pathological. I just can't seem to stop negative or distracting thoughts from popping up in my head. Mostly it's memories of silly little things, like minor arguments with people that make me feel terrible when, at the time, they were only annoying; or else it's memories of major embarassment that make me feel so ashamed that I physically flinch just at remembering them.
Sometimes, though, it's a sort of fearful fantasy that somebody or other will start a nasty argument with me, or will attack me or steal my things. Other times it's like a movie of all the bad things that could happen if I do something (like when I think about attractive girls I know, I will tend towards "fantasizing" about her rejecting me, even though I know this isn't good for me or what I want). Sometimes it's benign, but the situation makes it bad; I find it hard to concentrate when masturbating, for instance. Thoughts that have nothing to do with sex - like the fact that I have a doctor's appointment soon, or some fact a professor in class stated that I knew was wrong, or some stupid memory or argument that has nothing to do with my fantasy; and of course this often kills the mood.
I just have so much clutter going on in my head, but I can't find a way to just purge it all and be clear-headed. Is there something I can do to stop this, and to get clear-headed again? It's debilitating, particularly sexually (when I can't stop myself from thinking of the previous times my erection has failed, for instance, it inevitably fails again, and as such my sex life is miserable and quasi-nonexistant), but also in general - getting along with some people is difficult when I constantly remember and agonize over small arguments or comments, and concentrating on my work is very hard when there are silly little things flitting through my head. What can I do to gain control over my brain? How can I censor what stuff shows up uncalled for like that?