Ahh the bamboo, I remember getting that in an apple store when I was young. I used it until it broke then got another one because I hate change. Good times. It's a lovely Rin too, you did great.
I have some humble suggestions from a fellow photoshop user.
Layers: I'm not sure how you structured your drawing but I personally do my lineart then generally do the colour underneath it in seperate layers. This way if you mess up it's only one thing that went wrong.
Fill tool: It looks like you used the filler to colour large areas. Just be aware that it makes quite a sharp edge which often leaks over other boundaries. In yours for example you can see the hair red zone leaks past the lineart in quite a blocky way. You can also adjust the "Tolerance" value for the filler at the top of the window to adjust how much it spreads.
brush opacity: Photoshop is annoying in that if your brush is transparent (lets say 50%) then you can go over the stroke again and it will stay at 50%. If however you release the pen, then start a new stroke and go over it you will get the crossing section go darker. You can see this on yours mostly at the base of the hair points but especially on the eyes. The easiest solution is to make your strokes last as long as you can. I have in the past done strokes in different layers and manually overlapped them.
hair shading: I watched a
pair of
videos by Weee/Raemz many years ago to try and get better at hair and I found it very helpful. Get a round brush (or whatever you prefer) and in the brush "transfer" settings set the opacity to pen pressure, then you can fade hair in and out. This is of course, subject to stylistic preference. If you have a specific style you're chasing I may be able to give better (or worse!) help.
undo: You can increase the number of undo/saved actions in photoshop in "edit" -> "preferences" -> "performance" then the amount of "history states" is how many times you can undo. I've thrown mine up to 125 because I change my mind a lot.
With digital art I think the most important thing is knowing what is right and what is wrong. We have the luxury of infinite "undo" so it's more about going "yep that's okay" or "nope, I'll try that again" than anything. I sometimes redraw a line like 30 times before I leave it.
Just realised this is a resurrected thread. Big sad.