How to brighten a rainy day.
Rely on pictures of non-rainy days.
Read the kind of novel that got you into this business (or at least your historical speciality within it) in the first place. Actual quote from George Sand's Indiana (1832) (trans. Sylvia Raphael):
(you need to know that Indiana is the name of the novel's heroine)
Resist me now, gentle, trusting Indiana, for you don't know to what a vile brute you are willing to surrender the treasures of your innocence. Reject me, trample me underfoot! I have not respected the abode of your sacred modesty, I got drunk on your wines like a lackey, cheek by jowl with your maid, I sullied your gown with my accursed breath and your modest dress with my infamous kisses on another's breast. I did not fear to poison the rest of your solitary nights and to spill the effusions of seduction and adultery right onto this bed which even your husband rejected. What safety will you find henceforth behind these curtains whose mystery I did not fear to profane? What impure dreams, what bitter, consuming thoughts will seep into your mind and harden your heart? What phantoms or vice and shamelessness will creep between the virginal sheets of your resting place?Dude. It only gets better. "Have I not opened the door of your alcove to the devil of lust?" this speaker (who, as you might imagine, isn't making such a great showing for himself so far) goes on to ask. There's a restaurant called The Alcove in the next town over from my home village; I may not be able to think of it in the same way after finishing this novel. No, no, fair Alcove! I may think. Open not your door to the devil of lust! Or, on second thought, maybe I should frequent that place more often...
And give yourself a wry smile at the sheer sad weirdness of your neighbor's having e-mailed to say that his very young niece has just announced, via that magic medium Skype, that she is coming to England to see the two of you, and that the two of you are invited to come to visit her at her house, too. (She is not really coming, nor do you imagine you'll ever be going.)
Rely some more on pictures of non-rainy days--this time, of baby coots waiting for their mother to come back up from under water, bearing food.
Finally, go to bed and hope that tomorrow it won't be raining again.