Work related stress...Need advice!?

I'm a 24 year old wife and mother of 2 toddlers (2 & 3). I have a really hectic life. I work, go to school and have a whole home and family to take care of. I do the best I can to make sure my family is happy. I attend school only 2 days a week but I work sometimes up to 6 days a week. I need the money so I try not to complain much but my job is really starting to stress me out to the point that I get extremely bad migraines and stomach pains whenever I have to work. A few times I've even had to leave work early because I could barely stand or focus on what I was supposed to be doing due to the pain. I work in retail so it can get pretty crazy dealing with rude customers on top of a having an a**hole of a boss. She constantly schedules me outside of my availability which I cut back to 9am-5pm on weekdays so that I'm able to pick my kids up from daycare on time as opposed to constantly begging my mom to get them. She still schedules me until 7pm and sometimes 8pm. I'm free 9am to 7pm on weekends but she keeps giving me closing shifts (4pm-10pm) and scheduling me every single weekend although I told her that I could only work every other weekend due to me having no1 to watch my kids every single weekend and she agreed to stop. I talked to her about this several times and she gets better for a while but she ALWAYS goes back to doing it again. I can't sleep at night and my husband has been complaining about how distant I've been acting toward him. I am not my happy old self anymore and it's starting to strain my marriage. I can't afford to quit because my husband and I need the money because he doesn't make enough money alone to take care of everything yet. I think maybe I just need some time off. Everyone including 2 of my supervisors told me to see if I could get a note from my doctor and try to get a little time off to relax. I'm not sure if this is a good enough reason for a doctor to grant me a note although I'm really stressed to the point that my hair is literally falling out every time I touch it and all I do when I'm not working (very rarely) is sleep and cry. Idk what to do anymore. Is it possible to get some sort of documentation from my doctor so that I'm able to get any type of time away from my job?