Hey this is awesome! Really enjoyed what I played here:
The good:
Pretty good controls overall, nothing too difficult, and you've definitely fleshed out the game as a much more tight, mechanically-demanding platformer game with the likes of Celeste. It was easy to pick up and learn and you've tailor made good tutorialisation in the beginning to familiarise with the controls.
My personal suggestion is maybe you should make an auto-glide feature where if you hold spacebar, you will transition from jump into a glide at the apex of the jump? Consider playtesting with that and see how it goes.
Great art, music and art style that all blends in extremely well and gives off that gameboy feel. A cohesive artstyle is difficult to pull off and takes time to master and you've nailed it here! Awesome sauce!
To improve:
More often than not, I become more frustrated at the game, rather than at myself when I die. This is especially apparent with deaths related to the big spikes, consider decreasing and being a bit more lenient with their hitbox? ie. Dying feels unfair when its related to the spikes.
Also, I think the dying animation and their respective transitions could be a tad bit faster, this is exceptionally important because your game is centered around this difficulty, where dying is expected. Once again, think Celeste, their death animation is extremely fast, and the music never really stops (so the flow keeps on keeping on).
Anyway, awesome stuff here!