Here are my two cents for you as a beginner (seriously, this made me appreciate the game a lot more):
Find yourself a training partner. Any standard Samurai at the beginning will do. You need to get good at parrying. Yeah I know, I ignored that in everything From Soft before that, including Bloodborne. But in Sekiro it is the difference between “having a blast” and “dying repeatedly until you give up”.
After that you can more or less brute force your way to the top of Ashina Castle. You made it there? Good. Find one of these guys. There is one immediately after the first shrine as soon as you make it into the castle. Give him a name, I named mine Garry. Garry is harder to kill and his attacks come faster and harder. But they are still possible to read. See it as some kind of “advanced parry training”.
Do not dash forward. Do not get greedy. You don’t have to prove anything to anyone. Stay with Garry and train your parry skills. You need hours? Then stay there for hours.
As soon as you can handle Garry with ease, go into the very next room and tackle multiple Garrys. Study that blade. The next boss, at the very top of the castle, is a hard reset for many players. It’s kinda like Father Gascoigne in Bloodborne. He is hard, seemingly unfair even. But if you beat him, you can beat anything in the game.
After that boss you won’t get frustrated when coming across a strong boss, you will get exited!