@Vakinox ~It's been my experience, despite being quite terrible and presenting work that was likewise terrible, that if you continue to work at it and post edits showing your efforts, someone will eventually chime in with advice.  You don't have to have a straight up edit from a master to take advice and improve and even people with very little skill are willing to point out the things that seem off to them.
It's unfortunate that you feel the way you do Vakinox.  I feel as if I have grown immense amounts in the short time I have been participating in this forum.

It's a skill that requires time and practice.  Like any skill the best way to learn is to just do.  Look at how others did things and try to pick it apart.  Keep trying new things until it begins to work.  The members of this forum aren't obligated to respond to any threads, that I know of.
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On topic:  It feels odd to me that your weapon, armor, and other elements are all at odds with one another on perspective.  The anvil stands up with the well and the troll.  The helmet is sitting up at a different angle or is it laying down on it's back?  Now the armor is straight forward.
Maybe these are typical of the perspective/style you are going for. (I don't know)

Are you constructing the sprites within the dark border line around the outside?  The black border will eat up some of the value on the colors you are using which is partly why you are struggling to create curves and angles (and needing so many colors).

This took about 30 seconds.
You could always grab a selection by color of the reference images background ( white in this case) and then paste it onto a colored background to get a rough shape of what you are drawing. It's almost instantaneous, though it removes the aspect of really drawing the shape.  If you resize it and clean up the choppy edges from the resize and the .jpg pixelation on your reference image you get a solid blob of color shaped fairly close to the reference that you can then work on detailing without border lines.
Since you are working from such a close reference, it seems like a much faster way to get started.
Also...
I have been learning that working with black lines can be a hindrance. They immediately shadow the outline of your creation/piece and give it a flat/embossed look right from the start. [Sorta like a sticker with a shadow around the edge]
Working from a shape of color seems to work much better for keeping your mind wrapped around shaping the piece with colors rather than borders.
It's better to fill the inside and then trim the edges to look like the reference than the other way around, I think.

I did this cave before the mountain river.  I was going for a dirt floor and cool stone walls.  I came up with this brown/blue color scheme.  I'm particularly frustrated with the closest wall segment , but happy to take comments and criticism on any and all factors of the piece.

Is the color setup ok as it is or is the floor too far from the blue to feel acceptable to the imagination?
The way I did those crevices within and contours on the nearest wall feel way too contrasted to me.  Advice on how to fix it?  Is it simply a matter of adding a few more colors to lesson that sharp edginess between the colors?

Thanks ahead to anyone taking the time to throw advice or input.
Still taking advice on the river too if anyone sees anything that I can improve upon drastically.

Just some thoughts that might help.
Underwater you wouldn't see the actual flow unless there was something bending or reflecting light within it.
Below the surface it will appear sort of as the sky does though diminished. Flow would likely be captured with particles or elements within the water moving or drifting. If there is blurring from light within the water that is likely from variance in the refraction from light hitting the water. That's why shallow ocean floors have that very attractive rippling light effect.
Examples of underwater elements to simulate flow[bubbles, sticks, leaves, silt, plants along the bottom waving in the current.]
The look you have now gives off an appearance of surface shadows (top down/45 degree angle perhaps)  Try going for a transparency feel and put some other elements in.
I remember Donkey Kong Country 2 or 3 having really great water visuals.  Check that out maybe.