I am a fulltime social worker at a group home, during the interview I was told due to short staffing it would be required to work two 10 hour shifts on the weekends and when I started I was scheduled for 12 and have worked 12 hours on the weekends since. What should I do or can I do?

I don’t have it in writing but when I interviewed for the position I was told all of the staff (5 of us) would rotate weekends and that each of us would be working two 10 hour shifts each on the weekends we were scheduled. Well once I got my first schedule I noticed they had be scheduled on Saturday’s and sundays every other week with one other girl and we each work 12 hours each me being 7am to 7pm and her being 7pm to 7am. I initially didn’t complain because I understood from what they told me that it would just be temporary until someone new came. I started working mid January so I didn’t see the full schedule that included everyone’s work week. Well February came around and I finally had access to see everyone’s schedule and I noticed on the weekends I worked, I worked 12 hours with one other girl who was part time. And on the other weekend I would be off that weekend was split between the other 3 women one worked 7am to 3pm, the next 3 to 11pm, and the last one 11 to 7am. I felt that was unfair so I asked my supervisor about when the new worker was starting and she said soon. So fast forward to this month, a new girl just started and she basically filled in on the other weekend so she only works 8hrs. The girl I was splitting my 12 hour weekend with quit and today my manager had a meeting with all of us except one coworker and they basically told me I have no choice but to work the 12 hours until they get atleast 2 more workers. Then she proceeds to say if I wanted less hours I could go to part time when the next new person starts in April. I snapped back at her and said I was promised the 12 hours would just be temporary and that if I wanted to go part time I would have asked her that but I signed up for fulltime. So she responds with there’s nothing she can do. It’s hard because I am a single mom and my mom is of old age and my son is 4 so 12 hours an entire day two days in a row is a bit much on her and me as well since I only get one day off after working those 2 long shifts then work 4 days straight of 8 hours. I hope this makes sense I am still upset while writing this but I feel used and conned. I hate when people take my kindness for granted. I just got a new apartment so I need the income bur this type of schedule is running me dry and my mom as well since she is my only sitter. I guess I am mostly here to rant but also to see what I should do in this situation?