That was really neat! I really enjoyed the game, and it also surprised me as I thought this would be a slower paced game instead of such a fast paced platformer. It took me a few minutes to get used to the way the guy moves but you can get through some surprisingly tricky sections quite easily when you get used to the movement speed.
Only real complaints I got is that the combat could still use some work, all the bosses except for the last one where complete pushovers, the first one just died in seconds and the second one I could cheese by just attacking him from underneath the platforms. Sometimes it can also be hard to notice some small enemies like the tiny flying bug at the start of the fire level, but because of the rewind mechanic it's not too big of a deal either when you die to something like that. Additionally the quick step you get when double tapping the movement keys ended up getting me killed quite a few times, so I would suggest adding some way to change the input or have have the input be a bit more strict to avoid accidental usage.
Other than that I was surprised at how short each individual area was with each one only being around 1-3 "screens" worth of platforming, but I suspect that's just for the demo? The map function makes it seem like they are supposed to be bigger areas in the end. Despite that there was still a good climb in difficulty from each area to the next and they all seemed pretty unique too.
Keep it up dude, maybe you'll even figure out a name for the game eventually!