You're supposed to die and learn. Almost no boss you'll one-shot, even if you know what you're doing. Even the best players can't just know boss behavior without having fought it once already.

I haven’t played Silksong, but if the previous game, Hollow Knight, is anything to go by, it is very much Souls-like, which is what you’ve went through: if you lose to the boss, you get whizzed back to your last checkpoint, and all enemies, excluding special enemies, along the way get revived.

Again, if the last game is anything to go by, there should be various checkpoints all around, but you have to look around as much as you can with the current movement methods available to you. You’ll probably want to look for the closest possible checkpoint, and learn each enemy’s patterns to either avoid them, or defeat them without taking damage. Bosses are essentially just much bulkier enemies and have more moves in their moveset.

Souls-like games boil down to learning enemy movesets, finding ways to survive, and improving your timings to improve your survivability, while chipping away at your enemies until they die, and maybe along the way you’ll die a lot of times. It’s definitely not everyone’s cup of tea, but this sort of game gives many strong satisfaction.

It's a hollow knight game, not unicorn glade

Have you never played or seen the original? They're not intended to be a casual experience. Its a metroidvania with bosses intended to be challenging. So you unlock movement through the course of the game and have specific spawn points you get (in this HK games, its benches).

The thing with Silksong is that it is, in average, harder than its predecessor. The game is centered mostly around exploration, platforming and combat and bosses are a regular occurance.

I'd recommend trying out the original Hollow Knight and seeing whether you like the gameplay there or not, since it's a bit more forgiving.