You have good eye for poses and I liked your Katarina drawing, you also have a really good style, I think you should invest in personal work besides fan art.
I opened your image on my software and wtf 4800x2700pixel for a sketch? At least for me it's horrible to sketch in these resolutions, I advice you to work on smaller res to structure the figure first then you should size up your canvas.
About the perspective issues. Her horn is a bit strange, her left horn is like bending backwards and right horn in a straight line, some issues on the eyes and lips as you can see. Your selling point of the image (arm) is floating out of the void, see?
here I fixed her horn (a bit of foreground on her left horn) I don't know this character and what is the correct place to build horns (as your image have 2 different locations), but I wondered about constructing them them on frontal bone.
That thing on the tip of her left shoulder look like two fangs and her right one look like a folded up metal armor, so I guessed you're not on fang things.
Her arm needs foreshortening and you committed mistakes while trying. you can open up more canvas space to visualize whole upper body anatomy. its really hard to be accurate on something you don't know only by guessing it.
TL;DR
Study perspective, anatomy and main solids: spheres, cubes, triangles and cylinders. You can build your figure from these main solids, it may make figure construction and rendering way more easy. have fun ;x
