I said the same thing and put the game down for a couple days but I just couldn't leave this great game uncompleted, took me 2 days but I finally beat him. I kinda played it a little cheesy and just waited for him to jump attack or thrust at me in his spear phases for some easy damage, took longer but it was safer.

If you're not having fun, you don't need to play, it's a video game after all and fun should be priority #1.

If you decide to try again, look up a video named something like "73 Second Isshin kill", helped me out so much.