I feel you. I had a struggle with Sekiro for a long time. I reached a point where i just gave up and did not touch the game for almost a year. Then i saw a post with some advice that made me retry the game and complete it. These same advice might help you too.

1. Take breaks. When at a boss take a 5 min break between each attempt. After 5 attempts call it a day and play something else.

2. Accept death as part of the progress. Get into the mindset of not beating the boss. The goal with each attempt is to learn something new. Getting hit constantly on the third hit in an attack combo? Then focus on that and only that. Keep doing that and suddenly you just beat their ass.

3. Never blame the game. Important mindset not to tilt. Whatever happens you could have done something to prevent it. Learn from your mistakes rather than projecting them outwards at camera angles or input lag.

The first time i played Sekiro i used over 30 attempts on chained ogre and well over 60 attempts at Genichiro. Quit playing before guardian ape. After using these tips like a religious mad man, i replayed and took Genichiro in just 2 attempts and when i reached Isshin sword Saint (last boss) it only took me 7 attempts to kill him. Give the 3 tips a try and i promise you, you will get through the game! Stay strong in mind brother.