What to do when you're having a bad day?
The simple answer is: as little as possible. Meaning this is not the day to make any critical life decisions, call you ex or quit your job. Think more in the lines of doing what absolutely needs to be done that day, and save the rest of the time for yourself. Slow down and don’t spend to much time contemplating your life, today is not the day.
Another thing you can try practicing on a bad day is self-compassion, where you try to view yourself as you would a dear friend. How would you treat them if they were in your situation? Would you ask them to give themselves some slack or buckle up and quit whining? Be as gentle with yourself in troubled times as you are with others.
Let it pass. Sounds a lot like the «let it go» advice we always keep hearing (and maybe give to others ourselves), but there is a slight difference. At least they way I see it. Letting something «just go» can seem impossible, so try instead to let yourself be in the moment, but know that it will pass. Nothing ever stays the same, not even the feeling you have right now, even if it may feel like it.
Maybe these very simple ideas will allow you to accept your bad days with some grace and give more appreciation to the good ones.