You are torturing your body, no wonder you feel vexed.
While coffee keeps you awake, you need enough water as well.
From this post, it looks like you don't have a good management of
your time.
Plan everything properly. Set a time to do the leisure stuff, like
going to Bugis to collect things for today.
Know what you want to study for the day, prepare the necessary
things. For example, today, you want to study humanities. Pack your
humanities textbook, foolscap paper, stationery and TYS in your bag
first before going to Bugis get stuffs.
