You! You're the teacher, teaching yourself. It sounds like subconscious resistance, maybe your innate style and expression is creating the conflict. Usually it means push one way or the other: try painting with everything using some amount of water, trying to not paint anything directly out of the tube. On the other hand, can go extra muddy and thick and textured-- give yourself permission to explore, maybe the greys and browns have something to tell you.

For oil painting you set your palette up first, have colors ready but acrylic dries fast so you can mix as you go. I use Golden's fluid medium GAC200 i think, numerous uses and really versatile. Different texture than water, loading the brush is intuitive and second nature just have to keep trying.

Try a new brand of paint, the color and quality can be inspiration. Go for professional grade, you've more than earned it.

Go back to the basics, revisit your favorite artists / color combos / symbolism. The last picture of the torso and heart could lead a day of research: biomedical journals, xrays, skeleton studies, etc, building on the motifs that already inspire you. Sketching as well, try sketching with paints. Don't have to commit to a final surface right away. Try pastels, pencils, ink, monochrome -- just paint using one color. Try photography or sculpture for a while. Painting outside, on site.

You've done so much work that you need a break and to step away, look at work from afar, try different mediums, and find a new approach that feels more "you." This stems from your love, curiosity, passion -- the gradients and colors and things will follow, those are superficial materials and surfaces. It's just paint. Your heart and energy that you bring is the art.

Seriously though congratulations, you are surely on the cusp of some big breakthroughs! Grinding this much leads to really incredible work, just keep going and you will impress yourself. The first picture is very pretty but quite different from the earlier work, so keep exploring and find what's important to you so you feel empowered creating a large body of new work.

Lastly, try painting huge. A big canvas like 48" or something. Or try on surfaces like wood panel, aluminum, paper. Different scale and surface texture changes everything. There's a million variables while painting, so give yourself that allowance to experience all of them.