While it's a fun game, I do feel the difficulty is a bit off... the better you're doing, the easier it gets (you collect more buffs to get stronger, making later levels easier as the enemies aren't stronger/more numerous, this also gets you kills easier which heals you more), whereas losing resets you to nothing, which means you can lose easier as you're weaker, and could get stuck in a death loop, getting nowhere.
I feel it should start you on 3 hearts, so you could take some damage while running through the levels and this would carry over to the next, and then the very slow heal would make sense. If you lose, you still lose all your buffs which is rough, but at least you go back to full health. This would make it a fair bit easier, when it isn't that hard, but at least it wouldn't have the inverse difficulty curve where it's hardest at the start. It could then make later levels harder by spawning enemies faster according to level counter.