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?

Can you identify specifically why it is you mess up your initial values? For example, it's a common mistake when you're first learning values, to not establish your environmental values first. The value of the background and environment will naturally affect the overall values due to contrast. A dark background/environment will make even a midtone look bright and absolute white blindingly bright and vice versa.

You should explain your process for establishing your values. It'll make it easier to identify where the root of your problem is coming from.

>>7614685
I struggle with "artistic values", meaning I can read and replicate real ones pretty well and pretty quickly, but when it comes to something like pic rel, that's what takes me days. Something that doesn't adhere to reality but works AND looks interesting (at least to me). This is why the answer hasn't been obvious to me, since with these kind of low contrast values there are no definite rules.

>>7614697
oh that looks like its a mix of painting techniques but pretty tonalist. You'd still want to identify your largest value groups. That image is has two distinct values groups in the upper and lower half that anchor the rest of the values. Then it'd be a matter of selecting mid tones and sticking within those value ranges. Where something like chiaroscuro would be 0-100 for values, you'd be more like 40-70. Definitely harder to make work since it requires more subtle shifts in value. I'd recommend watching some tonalist master studies/painting techniques, it'd give you a good idea for how to approach and mimic this style.

>>7614684 (OP)
I think you're trying too hard to get it perfect every time. Realizing that you got it wrong and then moving on is ok. 100 practice drawings that are off, but are progressing towards better accuracy are better than 5 that you labored over to get perfect.

You can also design a warm up exercise to get your mind focused on value matching before you start drawing. Just pick a spot on a reference and make a quick swatch matching the value and make note if you were correct or not. Do this for 5 to minutes before you start your actual work.