I really don't have a mental slogan.
However what keeps me sane is being a atheist. Understanding Im not "being put through some kind of trial" that will all come out in the wash.
I appreciate being alive with an existence more than that of a tree or a flower; even though that involves being in pain. Living in our Non-Newtonian universe is a great place. Realizing what it is makes it even better.
Forget where I read this I think it was in Hitch 22 and was a Proverb from the east
When the axe came into the woods, the trees trembled and said "at least the handle is one of us"