When I work on a painting, it often takes me literal days to figure out how to handle values and even then they're not always great. How should I practice so that my values become much better?

>why does it take a few days?

I start with values that don't work at all, for example I make it too bright in the beginning. After making it wrong in the beginning, I remember it should be a bit darker, so I have to paint over it, then I figure out that the contrast is too high, so I paint over it again. In the meantime I play with the sliders, and after a few days I end up with values that are okay. That's the process of pretty much brute-forcing, how can I get better?