I started my masters, and I got my lowest mark ever of 55% in my topology final. My professor called me in to ask me if I had any issues in my personal life, which made it worse because I had to tell him that I just didn't work hard enough. It was compounded by the fact that he said multiple people got full marks in the course.
I think my ceiling was undergrad level maths. I'm pretty overwhelmed with the workload of a postgrad, and I think this isn't really for me, which is weird because in my undergrad, I thought that I wanted to do maths for a living.