No harm in calling to see if they have a cancellation. The appointment shouldn't last long, probably just a consultation. You'll probably be given the opinion as to whether you need a mastectomy or can have a lumpectomy at that time.
Maybe take a little melatonin to help you sleep or benedryl if that doesn't work.
This is definitely a lonely time in life, that's for sure. We've all been there. You want the appointment, but dread it at the same time. I had the cleanest house in town, because I wanted to stay busy. That really does help.
You can do this. Your faith will help so much!!! I found myself making a prayer notebook and praying literally for every single human being I could think of. I'd write their name down and what the prayer was for. It really helped me to stop focusing on me and remember that others also have serious problems. So happy you are a believer in prayer.