That’s common for beginners. If you want to give this a go, you should start by writing down fleeting, vague associations. “Something a bit sad or disappointing. A car. School and also not school. The texture of cinnamon rolls.”
It doesn’t matter that you can’t remember anything concrete at first. Eventually, you’ll remember more and more.
Even just writing down loose associations and your emotional state is enough; that’s how you get the ball rolling. Try it for two weeks even if it feels useless. Unless you’re taking antidepressants in which case this might actually be ineffective. I know this doesn’t sound worthwhile, but I know from experience (mine and others) that it usually works.