I have a few options, actually. The first is that I go back and reread a
manga I actually first heard about here on /co/ called Shinya Shokudo (Midnight Diner). It's about a little food stall in Japan that's open from midnight to around 7 in the morning, and centers around the stories that happen there. Over the course of thirteen or so chapters, you get to know the stall's regulars and their favorite orders, and you learn a little about the stall's proprietor, too. It's a bit formulaic, but it really is the definition of "comfy", so it's generally my go-to when I'm feeling down. Doesn't hurt that it recently got adapted for a Netflix series, if reading isn't your thing.
My second option is The Muppet Christmas Carol, which usually cheers me right up. By the time I hit "It Feels Like Christmas", I'm usually feeling better.
https://www.youtube.com/watch?v=3eSx9lxjfOQLastly, the nuclear option is Yotsuba. Yotsuba has never failed to lift my spirits, even when everything else failed. Guess there's a reason she's this site's mascot.
