Hey! it's a beautiful work in progress. I feel ya on the struggle with colour theory and light - it's something I only recently started wrapping my head around after winging it for many years. My favourite videos on this are from Marco Bucci on youtube - https://www.youtube.com/user/marcobucci
He breaks it down really succinctly. There are also many videos on texturing digital paintings that you might find interesting as well.
I think the thing that most jumps out at me is the use of the bronze-gold colour to do your deepest shading with - it looks like she has metallic gold paint on her back, which I don't think is what you intended. Given the warmth of her skin and the cool blue of the background, you would expect it to dip into cooler shades like burnt umber and greys. Though generally, you should be able to use an unexpected colour if the values and shapes still read, but unfortunately it's also gotten a bit murkey in that area. Turning the original picture to black and white, and/or squinting or looking at it zoomed right out and really small are all handy tricks to get a sense of the overall values and shadow shapes.
I feel like if you came back to this piece once you've done some study on light and colour, you would instantly see how to fix it, since you already have demonstrated a good understanding of proportion.
Also - using the lasso tool to shade is definitely an advanced choice, as you need to have an accurate sense of the shapes and values of the shadows in order to have the pieces read well, so don't feel too bad if you are finding it a challenge. Good luck!
