Should I quit my job?

I got an internship at a veterinary clinic to help me gain hours for my program to be a veterinary nurse. They knew I was in three summer classes and requested not to work a lot of hours, They only just now adjusted my schedule a tiny bit and still I am working 5 days in a row before my exam. From the time I started they have been working me the most out of anyone and would have me stay there until 30 minutes before my exams for chemistry which is my hardest class and I have put in all of my efforts into this course taking all-nighters as well to make sure I keep up because it is accelerated. I am at a 68 percent and I don’t even think I am going to be able to pass and even get into my program because of this place. They gave the only other worker a 17 day vacation in a 30 day period. I am completely shattered and devastated. I have worked so hard to get to this point and now it’s about to all be taken away. I have honestly been so severely depressed to where I can’t go on anymore.