It helps to think about one of my special interests (my favorite actor and his movies; I can replay movie scenes in my head, and that always makes me feel better). But some of my special interests involve doing something, and keeping track of things. If I'm depressed, that can be difficult. For instance, I like making Lego movies, but it takes a lot of planning and setup to make it work. I also enjoy knitting, but since I prefer to make things that are more complicated that just knitting a rectangle, it takes concentration. I can fix my mistakes if I'm doing okay, but if I'm depressed, I don't have the brain-power to deal with fixing a mistake.
I have a tendency to get into a kind of "stuck" feeling, which is sometimes obviously depression, and sometimes it seems to be something else. It's like my brain is powered by a big water wheel, but there is no water to get it going. When I feel like that, I want to pursue my special interests/hobbies, but I can't. That's not fun.