I thought I remembered reading something along the lines of the following advice in Littlewood’s Miscellany, but it doesn't appear to be there.
Researchers spend the vast majority of their time feeling frustrated. To improve the ratio of time feeling fulfilled to time feeling frustrated, whenever you find a new result or succeed in completing a proof, take the time to enjoy it, preferably by taking a long walk. Definitely don’t dive into the next problem, or go back and check the proof. There is plenty of time for that later.
I'm offering a chocolate bar to the person who can supply me with the true source.