How do I focus on studying when I’m having a bad day and have an exam tomorrow?

Take little steps and work through everything one bit at a time. Take breaks. Also generally: if you know that you can get a bad a day in a important time, prepare ahead so that if you have a bad day it won't affect your exam. Maybe you can talk to your teacher about that if it's a reoccurring problem so that you can maybe retake the test at another time. It's also important to remain calm. Do meditations or breathing exercises. And as stupid as it may sound rushing too fast actually makes things worse. Doing one thing at a time is good. And even if you didn't manage everything that will give you a better grade than rushing trough everything and then forget all once you're in the exam.

You have to try to get away from your problems, whatever it is. If you need it, take a relaxing bath, listen to some music or talk to a friend to calm down and then start studying effectively. It is obvious that with the mind occupied it is not possible to concentrate. Most likely you sit in the chair, look at the papers and your mind automatically goes away and you can not study, much less focus on what you have to do. So, as a conclusion, my advice is to do activities that you like so that you can first calm down; This will surely make it easier for you to study effectively.

Take some moments for yourself during breaks, dont rush in to finish studying everything as it doesnt help by cramming your brain one shot which when you're down you'll just let go of the knowledge you gain once you sit in your exam. If you know that you can get a bad a day in a important time, prepare ahead so that if you have a bad day it won't affect your exam. Talking with some teachers or close friends and family might help in letting go with whatever brought you that feeling as well as some encouraging tips to help. Most of all, remain calm, self care and try your best.