Get outside and walk
Walking is the one that works best for me. Get your boots on and head out, even if it's only twenty minutes. For me that's the hills around Hagley. Moving your body gets your head out of the loop.
Stick to the routine
On a bad day your head will tell you to skip everything. Do the basics anyway: get up at the same time, get dressed, have breakfast. Routine gives you something solid to hold on to when everything else feels out of control.
Eat properly
When you feel rough it's easy to live on rubbish and caffeine. Proper food won't fix everything, but running on empty makes everything worse.
Ask: is this under my control?
This is the big one for me. You can't control death, illness, losing a job or a pandemic. You can only control yourself and how you respond. If it's not under your control, try to let it go and put your energy into what is.
Catch the catastrophising
My mind used to go from "small question at work" to "I'll end up in jail" in about ten seconds. Now I try to notice when it's happening and ask myself: what's actually most likely here? It's usually nothing like the film playing in my head.
Talk to one person
You don't have to tell everyone everything. Just tell one person a bit of the truth. A different angle from someone else can take a lot of the weight off.
Read or listen
Self-help books did more for me than I expected, and audiobooks on a walk give you two tools at once. (My reading list is coming soon.)