That line, I don't have to believe everything I think, jumped out at me from Elisha Goldstein's 2010 post on PsychCentral's Mindfulness & Psychotherapy blog, and it still holds true.
Goldstein writes about a correspondent who, in the past, used to slip (like all of us) "into negative self-speak, hurling slogans at myself that I’ve internalized over the years."
Now, the correspondent says, "I remember I don’t have to 'believe everything I think.' I consider that maybe what I am telling myself is merely conditioned thinking."
This and other mindfulness practices, such as the body scan (you'll find an audio of one here), helps Goldstein's correspondent to ward off depression and to make better choices in the moment. To read the full post, go here.
By the way, it may seem obvious that you don't have to believe everything you think - but we keep forgetting this and treat our thoughts with great seriousness as though they have never been mistaken or given us a wrong steer!
As I've said elsewhere, our thoughts are not necessarily true and they're not necessarily important. So remember: I don't have to believe everything I think.