It's okay to check things out with reality but you have to accept your verdict! You check to see if your sister is yanking your chain or not, become satisfied that she is and your hair is not falling out and you move on. If you get the urge to "make sure" you tell yourself you already checked; if you have trouble believing that, when you check, you write it down, "Checked hair thinning at 8:53 a.m., Monday, April 16, 2012, none noted" and you don't let yourself check on hair thinning for at least another month :-)

Do the same with your other sticky thoughts; follow them through to their logical conclusion. If you find a problem, work on ideas how to combat it (just like you have by coming here and trying to get what others say not to stick in your head).

Jingle writers say that the only ways to get a song stuck in your head out is to either sing the whole thing through to it's conclusion or replace it with another song.

I know you said you like to sit sometimes and stare at the wall/just think; but if that's when you have the most problems, you can't do that and get rid of the problem, you are perpetuating the problem instead of getting rid of it. It's like being an alcoholic and not changing your walking route that goes by the liquor store.

Get more active, talking to others and doing things for yourself. "An idle mind is the devil's playground" :-)