This game is way too hard, getting the last heart in normal difficulty is already pretty rough, but getting the last heart in Hard difficulty is just stupid hard and flat out not fun.
Moving the circle with the arrow keys to aim just isn't very accurate, and you're being assaulted so heavily near the end of Normal and Hard difficulty you risk cramping your hand from mashing on the Z key so much.
One or more of these things that would improve the game:
Holding Z automatically rapid fires the projectiles.
The Hearts in Hard difficult come out way, way sooner. People who want to want to play for the high score can keep playing, but don't lock your story behind such a ridiculous obstacle.
Minor issue, while I did find it due to curiosity, it isn't really clear you well written moral degeneration story associated with your character when you view the art through the gallery that takes place progressively through all 3 difficulty settings. It seems like a "raise your score to get her to strip" game at first. Maybe make it a little clearer that the story is there.
Your art fantastic though.