Let me get this out of the way, cuz I'm sure you already know this: this art is fantastic. absolutely did a great job with the space, and I'm surprised a game with solid black background can have this much visual depth. This was a very pleasant game to look at.
The mirrors are both helpful and frustrating. They serve the role of a checkpoint, but if you don't wait for them to stop being inactive to actually use them again you die and return to the beginning. However I see now that this game was going for this bit of risk and reward with this and I think overall you did a good job. However, it took me a long time to ever encounter the mirror running out of revival time, so mayb that needs to be lessened a bit and make them instantly activate again when you revive as well as whenever you feel like click on one.
The platforming was entertaining as i really like platformers, however the game doesn't allow you to change your trajectory mid-jump. that would be fine, except that the game also requires lots of precision jumps. It feels like you were a bit inspired by jump king with this one. This is up to a developer to choose and for a player to decide if they like of course, and I respect that you did it the way that you wanted to with it. That's cool. Don't stop doing that. I also feel like something might be up with the hitboxes. The character feels substantially wider and their feet seems to clip on things. If the character is an unskilled person, that makes a lot of sense, but if they are agile and skilled (not sure if this is what you were trying to communicate) then their feet wouldn't clip, they'd make the jump. Because of the platforming feeling a bit "clunky" (again not a bad thing if that's what you're going for, and I have no room to act like mine doesn't also feel somewhat clunky) some of the sections I was feeling less like "oh i can't wait to beat this part!" and more like "when will i beat this part?" I hope that distinction makes sense. Again this is very much the kind of feeling some games want to generate. I'm not privy to you r design doc. EDIT: I would like to agree with the other commenters that when I did land the difficult jumps successfully, multiple times in a row, it was very satisfying.
To me, it feels like this game is begging to be turned into a game divided into numbered levels with a door you have to reach (maybe find and reach) but that aren't too long, where the challenge comes from different things depending on the level. Kind of like portal 2 i guess, which sometimes asks you to solve a hard puzzle and other times asks you to use the gun quickly and accurately. If that makes sense.
regardless this game does a fantastic job with it's art and music, the animations are great. and the platforming sections themselves are interesting and pose new challenges, especially with the difficulties provided by the "clunkiness" of the jumping i discussed above. I hope to see you develop this project further as I'd like to see what direction you choose to take with it, whether that be making it a "smoother" platformer with "tighter" controls, or if you lean heavy into the risk-reward gameplay, as both direction still offer lots of freedom to make it your own. Please update me (here or in our discord) if and when you update the game :) Hope some of this feedback was helpful.
(I did beat the game btw. good stuff! Great job on finishing it!)