Susanl, my dear, I was in the same boat. I started on Letrozole in April 2019 after a right breast mastectomy too. I finished chemo and radiotherapy by March 2019. 
 I just morphed into in old woman. I'm 46 now. Every thing ached! My whole body was in constant pain all the time. I could go for a walk for 45 mins but then struggled to climb the stairs when I got home. I bore this till oct 2020 then gave up. I was very depressed as if we have not been through enough. It even caused carpal tunnel syndrome in both hands. I got checked for arthritis but it was negative. It was all down to Letrozole in my case.  Spoke to my oncologist who then put me on Tamoxifen which I was told to start in December 2020. I could not bear to start it yet as I felt my body was so broken. I will start it in a few weeks but please speak to your Oncologist and see if there's any alternatives that they can give you. I will start the tamoxifen this week or next and as my oncologist has said some treatment is better than none. I'm just scared of the horrible symptoms flaring up again. Please speak to your oncologist and I really wish you all the very best xxx