How can I keep Krishna in my heart while I'm worried about my exams?
Hare Krishna! Thanks for your question.
In this life we're always worried about something; now it's exams, later it will be work, or family, or whatever. The goal of Krishna Consciousness is to always remember that Krishna is with us, supporting us, controlling the material energy and guiding our way.
Krishna is in our heart even if we don't remember that. We don't have to "keep" Him there—He's there of His own will. He will follow us around, birth after birth, waiting for us to remember Him and turn to Him for help. When we're in anxiety is when we depend on Him the most.
He will always be with us, so remember that and try to remain peaceful. Chant Hare Krishna and remember that ultimately He's in control of everything. Do your best and Krishna will do the rest.
I hope this is helpful.