Hi! I’m now 3 years on from mastectomy & LD Flap Recon, and am on Letrozole(Femara) too. Like you I still suffer from fatigue, also joint pain and insomnia. I also find I worry far more about things that, to others, seem unimportant. Could be partly due to the insomnia, but although Letrozole is a tiny pill it has far-reaching side effects. I try to console myself with the thought that it is (hopefully) keeping the cancer at bay, and I was fortunate not to require chemo or rads. My only advice is to go easy on yourself - hopefully the worst is over.
So sorry to read that the dreaded fatigue is still hanging on for you. As I am only just about 4 months past my last active treatment (mastectomy, 6 x chemotherapy, 15 x radiotherapy) I am, of course, still struggling.
Letrozole is also on my daily list of preventative medicines.
Having researched, it is possible in some cases, for fatigue to persist for a long period of time. Letrozole may also be the culprit. And, of course - we age - and with age periods of extreme tiredness can appear.
At a guess you may already have explored to rule out other possible causes - i.e. blood test and ECG?