This is a really creative idea of leaving behind ghost players and the fact that you have to do better is really cool as well.
I don't think the game's difficulty is that bad, difficult games like these have their place as well. But I do think some balancing is necessary especially for bullet speed.
I think maybe add some boost/power up after you've beaten your previous ghost, maybe an increase in speed.
The UI is nice as well. And the Music and SFx are cool too.
I think you could make the environment a bit better and have some props and maybe even walls and stuff to take cover in. Maybe have this game level based where this is the boss fight and you finish the levels before with the same mechanic.
All in all I really enjoyed this game.