The best way to combat a terrible day is to stay busy.
There are any of a million things you can do that we never think of. What you will find often is that doing something nice for another person when you yourself are feeling terrible has amazing affects upon your frame of mind, the catch is that good deeds and kind behavior will lead to the same in return.
Sounds like BS, but it isn't. Some of the most successful people in the world use tactics like this to stay positive, and to keep happiness and positivity in their lives.