The game is frustrating. Here's why:
1. The game is too punishing. There were many moments where I lost my lives in, say, level 3 and had to restart from the beginning. It's frustrating and adds no challenge whatsoever. If anything, just reset the enemies you've defeated for that level. That way, one won't have to get angry and put the game down or even delete it just because you won't let them progress.
2. The movement is just awful. Are you using forcemode impulse or something? If so, there are many tutorials on how to make an easy platformer movement system on YouTube. If you want to keep it as it is, them maybe implement something like a fastfall mechanic where is you press down, the character loses all horizontal momentum and falls quickly.
3. Lack of control over jump height. In many platformers, the more you hold the jump button, the higher the character jumps. the same goes for less holding. Again, there are tutorials on YouTube for these things.
4. Level 2. To the left of where you spawn, there is a certain platform that is way too high. I'm sure you're supposed to bounce off of the enemy to get to the platform, but what if one kills the enemy without knowing? You can lower the platform, or make a bouncepad or spring to gain height or just add a small platform in between.
Of course, you can choose not to listen to my rant, and that's ok, but know that video games were created to be enjoyed.