There are several different approaches you can use. As mentioned, health is important and you should go to the gym or exercise in some way, preferably with someone. Also reading about a variety of topics is also good. In that domain I recommend you read a history of mathematics, and in particular biographies of mathematicians. Some of them have lived very interesting lives and can be an inspiration. Norbert Wiener's "I Am A Mathematician" and Halmos's "I Want To Be A Mathematician" are two examples.
However, when it comes down to doing something for a long time, and especially something so intense like mathematics, you have to keep it fun. You can't take it too serious. Once you do that, you'll just poison yourself with self-destructive thoughts. "I'm not good enough" will only bring you down. The mind is capable of amazing things but you'll never realise your potential unless you start practising mathematics with a light heart. That can be particularly frustrating if you've just entered graduate school and you start to interact with people around you that are often faster and more knowledgeable than you. However, that does not negate the fact that there is enough mathematics for everyone, and that if you work hard enough and long enough in your own field, chances are you'll be able to do some interesting mathematics too.
Math is also a social activity. It's not something to be practised by one's lonesome in a dark room, and although isolation can help when working through problems, you should always return and talk about it with others. Pick the right people as well. Find the right professor or the right group of students who will bring you up and not enervate your soul. Whatever you learn you should release. It's not good to keep your math trapped inside you. Let it out to others and help others. Volunteer in the undergrad math help room(s) if such a thing exists.
Math is a field of logic but it's also an art. Don't always focus on mere logic and correctness. Rigour and proof is fundamental in mathematics, but once you have that, ask yourself, what is beautiful about this? Make the mathematics you do an art to be admired by yourself and by others.