not suitable to work

yesterday was my first day working as a waiting staff. i started work at 5.30 pm and ended at 10.15 pm. those hours at work passed so slow but when i finally ended my shift, i was dead tired. i didn't even find out how much i was getting paid per hour. anyways, the boss said i performed ok for a first timer and i was suppose to call her today at 4 pm to ask about the next work day. but the whole last night, i couldn't sleep thinking of all the consequences of working. first thing i didn't like about the thought of working is that she said we had to work 3 to 4 shifts per week summing up to almost 20 hours. that's even more hours than my credit hours in uni which is only 17 hours. i was worried i can't handle so much commitment at work because studies is still my main priority. then i thought, what if i still had to work as much during exam periods? knowing that i'm the type of person who studies last minute, the more i wouldn't be able to shuffle between working and studying. then, another main concern was how fast i could learn and familiarize myself with working at that fast pace restaurant. i thought that place didn't have much customers but apparently i was wrong. we were basically flying here and there during the peak hours. not only do i have to be quick and efficient, there's so much to learn. it made taking order seemed easy. somehow, the drinks menu was more ambiguous than the food menu. at least i know how to abbreviate the food menu, but there's like 4 fridges with so many varieties of drinks from soft drinks to tea to wines to beers plus coffee and other additional drinks from the bar. then i was tought wear to take the ice, milk, how to stock up the fridge, where to get the desserts, how to set up table and clear table, take order, serve food, remember table arrangement, clean cutleries, clear boxes, how to pack doggy bags, how to set up certain drinks, etc. a long way more before i could familiarize myself with so much things yet i only have one week more before semester starts. so i decided to put down the idea of working after i asked the boss if i could only work 2 shifts per week. since she can't let me work for 2 shifts or less, i decided to quit and not waste her time training me up. tada! that's the story of my 1st day at work. somehow studies is so important to me that i'm not going to think about work until holidays come. i'm not good with time management anyways. what's more, i'm dead tired after work so how am i suppose to do my uni homework or even prepare for prac?