How to quit my job without feeling like a horrible person.?

So here's the situation, I've been working at this restaurant for 3 years now, since I was 16. I've been promoted to manager. At first I though I had it all figured out with school and everything, but I'm ALWAYS working, I worked 11 hours today and all of my shifts are like that, I'm lucky if I get a day off and when I request one it's always a huge deal because they just "need" me, the main manager has no problem cutting out 6 hours early but when I want to come in an hour later on a day that's there's a full crew to sleep in a little, it's a huge issue. I never got proper training to be a manager, I'm constantly having all this crap thrown at me about how I'm not doing this and that right, but no one ever told me how to do anything to begin with and it's frustrating. They always leave me with the worst crew and it's just too much stress and I can't handle it anymore, especially with school starting soon, I'm going an hour away. I'm just overworked and stressed and I'm done, but I still feel really bad because the main manager and I do go back. She was one of my best friends through out high school, and I knows she'll offer me a raise and beg me to stay but I just don't want to get sucked into that because I know I'll give in and offer to stay, but I'm done. How do I just quit without feeling guilty?