quitting job??

I wanted to go back to work part time because I was losing my mind being home with three kids all day 7 days a week. So In march i started a new job that was supposed to be PRN, but it turned into them needing someone more full time and I said okay. I am supposed to be working 7:15-4:15 but I never get off on time. I'm always at work until almost 5:15-5:30. I'm working about 45 hours a week and I still have a 7 month old. I don't have to work I just enjoy it, but I feel so drained and don't want to feel like a failure if I quit because I have only been working for about two months. But I was basically told that staying an hour late is just how it is. I really don't know what to do.