If I tell you this is a phase will you agree? It looks like you had huge hopes and expectations from yourself earlier and something/incident happened that kind of drained you off all your energy. It could be you did not get good grades in 10th or you could not get the choice of subject you wanted that just sucked out all your zeal and enthusiasm. You had associated and attached too much of happiness with it. But life is like that you may not get what you wanted or expected..you are disheartened, hopeless...if I say you prepared only for success but did not prepare yourself for failure will it make sense? Think about it. You are only 18, life has loads of excitement in store for you in future..face bravely whatever is thrown at you ...give your boards with confidence and a smile on your face. You will find that.life is carving out new more interesting paths for you.
Next Steps
just focus on completing the exams. don't think of anything else. during the day when taking a break do something of your interest you could listen to good music. throughout the day.do at least 10 times deep breathing exercises. do some rigorous physical activity like running cycling if u Can. all this is just to release lot of sweat and toxins from your body.
Health Tips
write down your daily thoughts and feelings in a copy. and talk with someone a friend, a family member, or spend time with children it will change your perspective. all the best for your exams.