I can't.

I have a go to list of positive things to think about.

I have physical tactile things (a small rock I carry around) that brings me joy when I touch it because it reminds me of good times.

It is very easy for me to get stuck in negative thought loops, and no matter how many things I see / feel / hear / ... it doesn't get better (at least in the short term).

The question your asking to me is akin to "can't people control what they see" thinking it's like a movie you can choose to go and attend, when instead it's like "A Clockwork Orange" where in fact I do not get to control what I see.

My experience quite often is that if I get in a bad state, the things that usually bring me joy just no longer do. In some cases they even produce more sadness.