Sadness sucks especially when you need to feel motivated to revise for an exam you have tomorrow.
I know why I'm sad and I have gotten out of the environment that was making me sad but I still can't my emotional state back to default/normal.
just think of it like this if we didn't see sadness then we would not know happiness we would just be...well...emotionless and i get you i'm stressed about exams too but just think extended holz once they're over!!
You are sad dear, because you are suffering from a bit of stress and anxiety because of the exams. Once they are over, you will feel a sense of relief. Take the time afterwards to relax. Go for a walk, have a bath, watch TV. Do what you enjoy. If that doesn't work, then the best thing to do is to talk to your relatives and friends. Whenever I'm down, I hug my mum