This is an interesting idea! I like how you incorporated the theme.
Here's my feedback:
1. It wasn't immediately clear that I had to collect all the yellow balls before destroying my past enemy.
2. It could have been easier to tell the difference between bullets and the yellow balls.
3. It took me a second to figure out what the "shift" key did, but it adds a lot more depth to the movement.
4. I like that your past self keeps the upgrades that you chose.
5. It was so satisfying when I upgraded my power enough to get the tri-shot.
6. I found that if I went fast enough in a round, my enemy would run out of moves and idle so it would be an easy w.
7. The music is fire.
I made it to Round 14 before stopping. Overall, nice entry!