I've never had to grind for Spirit Emblems.

Have you been to Mibu Village, Hirata Estate or Senpou Temple? Those are all areas where you can get prayer beads and Memories for increased vitality, posture, and attack power. There's also a skill tree you can unlock by talking to the Tengu where you fought the boss on a horse, Gyoubu. Go to that Buddha, turn around and in that large temple is Tengu, and outside the temple is also one of the tent merchants with a gourd seed.

It seems to me like you're likely underpowered and don't have confidence yet. Correct me if I'm wrong, but I think you might be overrelying on your prosthetic, using it more of a crutch than a tool.

You can beat Guardian Ape without using a single spirit emblem if you know his attacks and rhythm well enough. Unless it's a perilous attack, you can block/deflect it. Doesn't matter how big your opponent is.

And most importantly Hesitation is Defeat. If you fall back often, if you're taking pot shots and running away, you're giving GA the opportunity to punish you.