I am suppose to have a mastectomy. What should I expect?

Emotional upheaval. It is a major event and comes with a lot of emotional upheaval. Talk to your pcp, breast surgeon and if possible also a plastic ; reconstructive surgeon as knowing the possible options for breast reconstruction and seeing some results in advance will help you through this event. Good luck.
Quick recovery. And normal activities , with out hindrance , any thing you did before you can do after the procedure . You will have options of reconstruction of breast same time or after completion of treatments . You will be fine , don't get depressed speak to your surgeon .