Tamoxifen or Letrozole???

I was 53 when I got bc. I had been on the pill, to keep my periods regular and yes they were perfect at that time. I was not menopausal. The doctor told me to go off the pill as it was feeding the cancer. I had only one more period and that was during chemo. I never had another one. My oncologist put me on Tamoxifen for two years and i have just started Letrozole.