Think about something else. Or nothing at all.
It's that simple.
We're capable of one thought at a time - often in rapid succession, but only one thought can occupy our minds at a time.
So, quite literally.. it's as simple as thinking about something else.
The very best thing to think about, always, is the present moment. Focus on where you are, what you're doing etc and you won't be thinking about something else from the past or be worrying about something that may or may not happen in the future. "Be present," is some of the best advice I've ever been given.
Alternatively, you could think about nothing. i.e. do meditation and just clear your mind. Try the series of guided meditation lessons from http://www.freemeditation.com
But if you're the type that prefers some sort of therapy, or would like to try it to see if it works, I got a lot of value out of reading Feeling Good by Dr. David Burns. It's about 700 pages but very very valuable. It's basically a how to guide to self administered written Cognitive Behavioural Therapy to treat depression/anxiety/ocd etc. I got a lot out of it a couple of years ago.
_________________
No

for supporting trump. Because doing so is deplorable.