Hi, this may get long, I will do a TLDR.
I started working for Chessington as a bartender in October and on my contract I am only meant to have 15 hours (I don't know if even that is too much) but yesterday my manager put me on the rota for 44 hours next week as it is very busy. When I went in to work I had the intention of fixing it but when I went to talk to her I sort of froze and I couldn't put me foot down on the issue. When I said I couldn't work that long, she sweet talked me, saying it is because I am very good and she trusts me to be able to get my head down and deliver excellent service without a manager looming over me but I hardly feel that way most of the time. I asked if she could at least cut it down to 25 and she said she will try. I still feel even 25 is too much. That is equitable to working 2 full time jobs back to back. Additionally, I am sometimes working 10 hour days on saturday or sunday. This is my first job and I haven't had any guidance on how much I should be working whilst doing my a levels which is making me panic and I also don't know what steps I can take to make sure my hours stay down.
TLDR: I am not assertive enough to tell my boss to keep my working hours down to a reasonable amount, but I don't really know what a reasonable amount is.