Pursuing a PhD in math has been my goal since high school. Going full speed toward this goal for the past four years has taught me that you have to be careful if you want to avoid burning out. This is especially true if you have similar goals as mine and load up on as many math courses as you can possibly handle at once. The spring semester of my second year was especially rough, to the point where I sometimes questioned why I was doing this in the first place. If I could go back, this is the advice I would give myself.
Always make time to remember why you love math. For me, this takes the form of informal learning. I'm primarily interested in applying analytic tools to problems in number theory and combinatorics. So, consistently engaging with these areas, even when my courses aren't directly or at all related to them, is how I remind myself why I love math. I maintain this constant exposure by going to seminars in the department (Ramanujan-Serre for number theory), online seminars (ETA for ergodic theory), having a good community of math people/friends to talk to (math club or just studying in Kerchof), and picking up random math books every once in a while (I've spent hours exploring the hundreds of books in the math library at Clark). I also try to fit in a course that is directly relevant to my interests every semester. As a result of doing these things, I have an ever-growing list of math topics I want to learn, so that I always have something to be excited about. One of my favorite parts about being on a school break is that I finally have the time to sit down and learn what I didn't have time to during the semester.
Building on the last point, courses aren't always where the most interesting or inspiring math lies. I've found that it helps to actively seek out other sources of math to stay motivated. A lot of the time, the courses you take are purely about tools, but tools on their own aren't always the most interesting thing to study. Applying tools to interesting problems is often more exciting, but you might have to go beyond the scope of your courses to find such applications.
Finally, take care of yourself. I used to think I could get away with not doing this. During my second year, my sleep schedule was nonexistent, and I was an anxious wreck. During the second semester, I was pulling an all-nighter every Thursday. This ruined my health and made math much harder as the semester progressed, as I was becoming more and more of a zombie each week. As simple as it is, my advice is to set a realistic schedule at the start of each semester: decide when you'll start and stop working each day, aim for consistent sleep, make time to exercise, and do your best to stick to it. It probably won’t be perfect, but having a plan helps you get back on track faster.