Upcoming mastectomy - any tips?

I hope that your surgery has gone well. I had what sounds like the same surgery, February 2020.  Mine went well, the drains were bothersome, especially when changing position at night. I was so pleased when the drains came out, on day 5 and 6. Do your exercises. Although I found being Right handed, I used my right arm instinctively and relatively easily. I never needed strong pain relief, but everyone is individual. Take what you need to maintain movement and do exercises. I didn't get on with the little horseshoe pillow. I found great comfort in having a normal pillow on my left. When on my left side, I'd put my Right arm on top of the pillow.  I also used the pillow behind me when laying toward my R side. It was quite a while before laying on my Right side became more comfortable. Bed time was when I needed any pain relief the most, usually Paracetamol. Don't be surprised if you have or get a seroma. Apparently these are quite common, and resolve at their own pace. Everyone's experience is individual. I'd love to hear how you get along. Stay positive.  Experiences vary. Do you have Chemotherapy to follow?