Hey @juliemillardart Ideally for me, I work out my values early on in the process by doing small 3 value, black and white thumbnails and working out the big shape design before I dive into painting.
One way you can kind of mimic this process if you're already deep into a piece (as you are here), is to create a white layer and a black layer, with a mask, and some transparency and on top of the image. You then work only in the masks to figure out your big shapes. I suggest working small so you don't get into the fine details. You want to just focus on a few shape areas. This also doesn't instantaneously solve all your problems, but it does give you some direction. Often with values, there is a tendency to go too far in the dark or light direction and we end up with a kind of "noisy" image. Really, the range of dark and light is much smaller than we usually think in any particular are. So for example, the shadow on the snowman could be lighter overall than it is (that's just one example).
I don't know if that makes sense, but values are such a huge subject, so it's challenge to try to put it into words.
I've attached a photoshop comp that I quickly put together with the black and white layers so you can see what I'm talking about. I use this trick and variations of it sometimes when I find my values getting out of control. You can see how just doing this quick "study" helps to group and design the image into big shapes and makes it read more clearly. I'm not saying this is the right grouping or values by any means, it was just a super quick pass.
Add the fact that the composition of your line drawing is quite solid made it really easy to just throw this on top, so you're not as far off as you might think!

color-adjustments.psd
Let me know if that makes sense and if you find that helpful at all!