For me, when I spend too many mental cycles thinking about any problem, the problem becomes oppressive. It could be anything from the fact that there's a spider in my living room (I'm afraid of them) to some seeming great personal injustice to everything that is wrong with the world..

My solution, learned from a psychologist friend, is to note when I am investing too much energy in a problem, take a break from it, and then approach it again when it isn't so important (if possible). With practice, it's become easier to pay attention to my thoughts and cut-off when they start spiraling. Very few problems need to be dealt with immediately. Some even solve themselves over time or turn out to not really be problems but truly opportinities or motiviations for change.

Then, of course, there's the prayer:

Higher Power (God, whatever you like) please grant me the serenity
To accept the things I cannot change,
The courage to change the things I can,
And the wisdom to know the difference.

(I'd attribute this prayer if I knew who wrote it!)