The difficulty is too high
(no I did not work on the game but I do like it and the current state of the game is good)
I made it to stage 14 without losing a single life, the difficulty is not too high, and this is an arcade style game with increasing difficulty and no end, so saying "The difficulty is too high" doesn't even make sense
but if you look at the options you can see there are cheats available, they disable the high score but they are also fun to mess with,
if you want to know what this game is based off of go play the Galaga or Galaxia arcade games, they are super easy to find online
Galaga and Galaxia, plus most other shmups, have vastly smaller hurtboxes than this game though. Not as small as bullet hell games, sure, but still way smaller than this. I used to play shmups and bullet hells, and I've definitely played shmups/BH games easier than this.
Half the reason this game is as hard as it is is because the player hurtbox gets absolutely gigantic in the later sizes. The other half is because the game's super cramped, I wish the dev would extend the screen up a bit and make it a vertical game screen ratio like a lot of the vertical shmups out there.