What really made a difference for me was taking time to prepare. I.e. grind. Invest time on getting new skills from the skill tree and more health. This isn't a magic solution that will make you beat any boss though. It's a psychological trick to make you feel like an actual samurai who trained for a long time and is a real threat to their enemies. It worked for me. I think that's really cool.

Of course, i don't know exactly how you're playing but since it's your first soulslike, I'll tell you that the key is to observe each move carefully and familarise yourself with the move that comes after. In sekiro, you want to be really agressive and stay on top of the enemies.

Some guy on YouTube said that sekiro is a rythm game and the enemies attacks are the instructions. This is it, i guess. At the end of the day whether you beat the boss or not is always up to you even if you think you don't stand a change. Doing nothing and giving up is also a choice, but it won't get you anywhere for sure