I know exactly how you feel as I had double mastectomies with expander implant reconstruction 18 months ago. When I went on holiday 9 months later, I was shocked that I was unable to stay up in the water as my pectoral muscles are not how they were pre-BC. I have since started doing rehab Pilates under supervision and that has made an enormous difference to my upper-body strength. I would encourage you to find a similar service - even if just for one session so that you can go home with specific exercises recommended by a qualified rehab physio.
Good luck,
Hugs - Jeanine