I am sorry to hear. It was a shock for me too when I was told I needed a single mastectomy instead of a lumpectomy. I have however found a lot of positives in having a mastectomy and have decided to have a double a mastectomy. I stained flat and love it. I am comfortable and recovery was beautiful.
I am happy to answer any specific question you may have. Here a link on staying flat that may help.
https://notputtingonashirt.org/Good luck! đź’«