I’m going to rip off/adapt some questions Byron Katie uses when she’s trying to change people’s limiting beliefs…
1. Do I know [this thing I’m obsessing over] is important?
2. If so, how do I know it’s important? (List reasons to yourself.)
3. Is it possible that it’s not as important as I think?
4. How do I know it’s not important? (List some more reasons.)
I also find it helpful to step back and get some perspective. If I’m annoyed with something a friend did or said to me, I remind myself how lucky I am to have friends at all and I try to think of a time when I had a lot of fun with that friend. If I’m sick and tired of a prof or class at school, I remind myself how incredibly privileged I am to be attending school at all, and I mentally re-play some of my happier memories at school. That kind of thing.
Also, mindfulness always helps! Just take deep, long breaths, focus on your breathing, stare at a fixed spot in front of you, and feel all the physical feelings of your body, the ground under your feet, etc. You can count to 100 in your head if that helps you. Then shake it off and go about your day.