Hey!
Sounds like you're having a hard time at the moment pal, I can see you've made 3 posts today about your low mood. You need to speak to someone about how you're feeling, you may be experiencing some anxiety which is common just before exams. The fast heart rate may be due to this but most importantly you need to speak to an adult that you trust about how you're feeling because it won't go away on its own, this could be a teacher, parent, family member etc.
Are you looking after yourself at the moment in terms of sleep, meals, personal hygiene? These are all fundamental to wellbeing, so if you've been neglecting any of those then go ahead and practice some self care. I would argue that caring for oneself is much more important than the outcome of a result, you'll need that skill for the rest of your life. Exam results aren't the be all and end all so if that is playing a contributing factor, keep that in mind.
I hope things pick up for you mate.