Waiting for first appointment at breast clinic

Hi I'm new to this site. It looks a very supportive forum and I know will be better than my incessant googling. I'm aware many of you are much further along the journey. I have a sizeable lump in my right breast, I'm 48. My GP said she was hopeful it is a cyst as it felt circular but that she would keep her fingers crossed for me.  It's between the size of a walnut and lime so pretty big and at the bottom to outside of my breast. I'm obviously terrified that because of it's size if it is cancer it won't be good news. I'm finding the waiting absolutely unbearable and I'm struggling sleeping. Because of its size I find it really hard to not think about and it also feels very achy. 
My apointment is on the 21st December and I understand they will do biopsy and mammagram etc and then it will be around a week for results - depending on Christmas/ New Year.  At the moment I'm thinking of every possible worst case scenaro. I'm always a very pro active patient and often an advocate on behalf of family members and feel so helpless at the moment. In terms of questions to ask on the 21st any pointers would be appreciated. Also any top tips on how to switch off a very worried mind. Many thanks and sending much love to you all