I think the first thing I do, which has taken me a full 32 years to learn is to never judge yourself on your worst day. It can be so easy to send yourself into a long-term depressive episode just because your one day sucked because you didn't get enough sleep / are sick / had a rough day / etc. I remind myself that today is just one day, and there are many more ahead. I accept the fact that my day is not going well, and just look forward to tomorrow, when it will be better. Under no circumstances should you dwell on how bad today is and how that means your life sucks. That is the path to perdition.

Instead, accept it, treat yourself, and exercise self-care. It's okay to skip things or put things off (if there are no deadlines to meet of course). Just enjoy and pamper yourself.