Get something to write with and some paper, preferably some kind of notebook or journal with lines. You can also use your phone notepad but I find it helps more to have a physical Thing I can hold in my hands for this, it makes it feel more real. Maybe that's just me.
Write a list of things you like. It doesn't matter what it is, how stupid you think it is to like it, how abstract it is - any judgements you have about it are moot. It can be as simple as "blue" or as specific as "episode 5 season 6 of my favorite show paused at 15:10". You like it, you write it.
Expand this list every single time you think of something new. It's okay to write the same thing twice on accident. Just keep expanding the list. This works best with a lined notebook because you can ideally write one thing per line and fill the book overtime. Feels like making progress.
Here's the most important part; every time you feel like shit and can't think of anything good, you can look in the notebook and remind yourself that sometimes things aren't shit. Sometimes you have a funny moment from a show. Sometimes you have a cool game that sticks with you. There's things you like, and they exist no matter how you feel. Just because you can't remember the joy it gave you doesn't mean it never existed, and the list is proof that it made you feel something good. Maybe it won't make you feel immediately better, or even happy, but you'll remember that it's possible to feel that way again.
This is one of the things that got me through my worst years. Hoping it's of some help. If not, at least you have something to do.