(disclaimer; if my english comes out unintelligible it’s because tired people shouldn’t write but i can’t help myself.)

completely blown away by this painting. you have a fan. painting such a classic in perfectly, imminently modern way is a feat. your ability to revel in symbols yet never letting them take the center stage away from the human, the life, is amazing. the strong physical presence through color in cloth, in flesh, is wonderful!! the quality of swirls both synthetically perfect and organically recognizable speaks of hope, aspiration and destiny. the composition keeping it all moving is poetic and deliberate, inspirited and intelligent.
it’s a beautiful, beautiful painting!

one petty, jealous remark: the swirls in front of the neck should recede and separate much more, but then you’re not finished there. for now they appear to define the shape of the neck.

how about googling ‘mummie’, ‘mummification’, related words to find images of skin stretched over skeleton?
for hair, sorry, beginner here, but for dark blonde hair a quick test with titanium+ochre+umbers should point the direction. raw umber for shadow minus one step; more ochre no umber plus white for highlight; burnt umber or transp oxide red for turning the shape in light. earth colors were recently deemed a kiss of death for paint film integrity and longevity but you can search the web for many different ways to work around them, if so inclined.

it’s no longer allowed to speak of zinc white, but a mixing white would work wonders for luminous transparency in hair. layering, scumbling does too, of course. the easiest thing to do would be to test on another small canvas, or oil paper, to work with no stress on experiments.
i wish you endless inspiration!

1. I’m really struggling with the monster/eldritch horror in the background, because I’m having a hard time visualizing something I really have no reference for.

In this case, unfortunately, you are a bit on your own because it is an artistic decision-making thing. You’ll have to go looking for references that you like, and the things you may wish to incorporate might not all be from a single source. And you’ll also probably just make some of it up.

Think about your original concept and why you want the character in there, what you want him to look like, and do your search based on that. Video games have great creepy characters too, so those types of images might be a good place to look.

If you are going back and forth between a few ideas and need to visualize them, do it in your sketchbook.

2. I have light brown/very dark blonde hair, depending on the lighting, and I’m really having trouble finding the colors I need to use for my hair.

Go back to basics, earth colors, and work dark to light. Black (just in select areas), burnt umber, yellow ochre, white. Maybe a touch of an earth red somewhere too. But you can do most of it with burnt umber, yellow ochre, and white. You will have to make a decision about where your light source is and go from there.

Keep posting updates! it’s a cool sp.

I wouldn’t spend too much time on the demon’s eyes unless you want it to be the center of interest. Unless you are going to add more detail to your portrait of course.