Thanks everyone for the comments. The game was designed to be hard, but some of the sections have been more difficult for people than we expected. We have an update in the works we're planning to release after the jam with some changes, with one of the big ones being that jumping next to a wall no longer requires you to push against the wall to restore your double jump, which should mitigate a lot of the frustration in some of the levels people have been struggling with, like Tricky Walljump 2.
And just to be clear, the bonus levels are specifically designed for experts and are not intended for most players to finish. The difficulty level is significantly higher than the main levels, to the point where beating them is pretty much out of the realm of possibility for the average platformer player. If you beat all the main levels, you already did an excellent job, so don't stress too much about the bonus levels! That being said, they are not impossible, and I've beaten them all a few times just to ensure that. If the looping music is getting a bit grating after a lot of grinding, you can press M to mute it.