Hi Larae while I didn't get a mastectomy, I had a lumpectomy and the one thing I wish I knew leading up to it was practice sleeping in another position, been a stomach sleeper my whole life so when I got my aftercare instructions and it was said back sleeping would be best, well it was impossible. I was up for 3 nights and finally had to figure out with some pillow positioning how I could sleep on my stomach without any pressure on my breast. I still wasn't very comfortable and only was able to get 3-4 hours of sleep at night for a few weeks.

Same here that I had a lumpectomy and not a mastectomy but I did buy a “mastectomy pillow” and that has been a god send to sleep with as it keeps me on my back while my incisions heal as I’m a right side sleeper usually! You can find them on Etsy and it’s truly helped.
The other suggestions of pre planning meals and keeping lifting to a bare minimum is key. I also bought some full zip hoodies that had pockets for drains or tissues or TBD and they have worked out wonderfully too!
Hugs being sent your way for a speedy and pain free recovery!

Get all your errands done- groceries, prescriptions, etc. because they may not release you to drive right away.

Pick yourself up the ingredients for easy meals, or microwave items, and take-out menus! I cooked ahead and froze things, so soups and casseroles could be assembled quickly.

Clean the house, there are restrictions on lifting things as well as "repetitive movements" like sweeping, mopping, running a vacuum. I put pet food into smaller containers- again, a weight limit on lifting. And made sure all the laundry was done.

Move things you will use- to mid level range. You will not be able to reach very high at first (so my casserole dishes came down from the top shelf) I also made room on the counter for some of my pots so I would not have to bend down, and move things around to get to what I wanted.

(It sounds like a lot, but it all helped me use the nervous energy!)

I had a recliner- so I planned to sleep there, but pillows to prop yourself up will work. Get yourself some books, music- whatever to relax with!

Clothes- have an assortment of button or zip up shirts, etc. My fav was my hoodies.

Ask if the hospital will be supplying any drain supplies. I was given a small U-shaped pillow to go under my arm which kept the pressure and friction off the incision site, as well as a small rectangular pillow to go under the seat belt. Just in case that area was uncomfortable. The pillows were sewn by volunteers. I was also given two mesh aprons, each had two pockets to hold the drains and they tied around the waist. Being mesh, I could wear it in the shower, and still have a dry one to put on afterwards.

I am sure others will chime in with more suggestions! Keeping you in prayers!