If you didn't get genetic testing using the excised tissue, it's likely not too late. I had the OncotypeDX test (of 21 genes) and the result was a 3% 'risk of loco-regional recurrence within 9 years' if I took an aromatase inhibitor or tamoxifen. That translates to a 5% risk if I do not using the current thinking about estrogen-positive breast cancer that adjuvant therapy can reduce the risk of recurrence by about 42%. In a case, I didn't think that the risk of faster bone loss or possible change to endothelial was worth the 2 percentage point reduction in risk. But that's a very personal quality of life decision as I'm older and not from a family of people who routinely lived into their 90s. And most died of heart disease so elevating cholesterol when mine cannot be controlled by statins got my attention.

It really has to be a personal decision about what you want to do for yourself. I would suggest doing as much research as you can, including asking questions of your health care team. Then put it all together and decide what you think you can live with. My problem is that my husband and children are scared of losing me and are mostly focused on the cancer. But it will in the end be my body that suffers the consequences of whatever treatment I do. It is just a really hard place to be in. Good luck with everything!