Reward yourself. Doesn't have to be anything crazy, even the tiniest thing that most people find insignificant, but that you really appreciate, is good enough.
A cold coke and a movie / tv show? Talking to an old buddy? Dust off a personal project or start a new one?
The last one can get tricky, because it can translate into work directly, which isn't overall a bad thing. It just depends on personality. I know people where "keeping busy" isn't really a problem solver, it just postpones the problems instead. I personally find it liberating, because sadness to me isn't an actual problem. It's a state of mind, and if I can derail myself from those thoughts, I'll eventually never go back.