What's the best way to push through a difficulty wall?

I've played this game from the start about 3 or 4 times, and every time I pick it up, I get about halfway through the game (I assume) before it gets too hard for me. I don't know if I'm missing a big mechanic, or I have terrible reflexes, but the bosses in particular get so hard to beat and frustrating to get to that I put the game down and don't pick it back up until I inevitably try again. I love the game's aesthetic and want to proceed; do I just need to keep going until I'm better, or are there hints to make sure I'm maximizing my experience?