Some notes:
Your problems are multiple, so lets’ take them a step at a time.
Composition - your painting is effectively cut in half because of the black wall between the figure on the left, and the Medusa figure. One of the purposes of composition is to lead the eye through the elements of a painting, and what happens when you look at this, is you look at the left figure, and the eye stops. If you look at medusa, the eye goes from her, to the statue, and out of the painting following the sword. How you place elements leads the eye through the piece, and you should rethink this painting with that in mind.
Anatomy: You can’t fake anatomy, and you have some study to do. Nine months is barely getting started, some people take years to get comfortable with drawing the human body without reference - and making up alien anatomy is harder, when you’re mixing human and fantasy. You either need to find poses for this, or keep your work more simple as you learn. I know you’re ambitious, we all are when we start out, but there is a real reason to slow down and do studies and learn to draw from life, before you can make it up. Even the masters would do months of studies for a painting like this, posing models, trying out different compositions, and taking parts and exploring them in detail, getting more familiar with the subject, before they committed to the final. The scale of the figures is off, too.
Color: this is where the bulk of your flatness is coming from. Even if the ambient color is green, there is going to be more color in the scene, and you can use color as a compositional element as well - using darker, more cool colors the further back in teh scene you go, to great the illusion of depth, and rendering the figure on the left with real skin tones, warm tones with cool shadows and the ambient light as a highlight, and the warmth will offset the figure from the medusa figure. All three figures have the same color and values (greyscale), putting them on the same plane of space, and that flattens the painting.
Focus on these aspects, for now - there’s issue with texture and detail, but that can wait until you’ve mastered these issues. It’s all part of becoming an artist, and what you learn in school and working later. For a painting like this, look up some of the masters, to see how they handled the issues you have.
I think this may be too ambitious for you, right now. It’s okay to try, but I think you need some more focus on fundamentals before you try something this complicated. Break the painting down to pieces - explore each figure, in terms of pose and detail and anatomy, explore rough compositions of teh whole piece. Do some studies of Medusa’s head, then her body. Work on this as a line drawing, and get the composition worked out, then add color and texture.
Hope this helps.