Really cool game! I always love pixel art games, especially when the art is really good. I like the gameplay and I respect the tutorial. A lot of games I play lack one so this makes the game feel way easier to get into. I noticed that whenever my bullets were not empty there were these black bars over them. 
They disappeared when all the bullets were empty. 
I also agree with the variety of enemies between levels. If you decide to take this game further there could be different enemy abilities such as firing bursts of bullets or firing heavier bullets that take more hits to destroy. I love the detail of the cowboy hats on the hearts. I thought that was really funny. Overall, this was a really fun game that I enjoyed playing a lot. Good job!