My work gave me two 12 hour shifts after changing my availability to weekends due to school and an internship

This is a job I've worked at for 5 years, it's a fast food place and has never been good. I quit last year but just started it again after a couple months. I have another job/internship now I've always wanted already. Should I walk out? I'm scared because my new job is nice and I don't want them to fuck it up. They've always mistreated me so i kinda don't feel bad but I like the nicer people there too. What should I do?