What makes the PhD so difficult?

Hello, I will be starting graduate school in the fall and I really want to know what about the PhD makes it so difficult. I often hear that it’s not even the coursework or research that’s hard it’s more so the bouncing back after failing so many times that makes it mentally difficult. In your opinion what has been difficult for you? I often worry about if I’m smart enough to do a PhD.

Edit: If you vote “something else” please comment what that is 🙂

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