Your oncologist is best qualified to know what is best for your particular situation. While tamoxifen can be of benefit to reduce a person's chance of recurrence, if your oncologist thought that for you the risks out weigh the benefits then feel reassured that you are following the best medical advice.
As we all know, there are no guarantees with any treatment, but if you do all that is recommended then I think you can feel comfortable with that. None of us want the cancer to come back but there can be so much to consider with any treatment that we are best guided by our doctors. Maybe you could contact your oncologist and just check with them about the idea of trying it for three months just to put your mind at rest.
I am sure that your breast care nurse was just trying to help but it is a shame that it has unsettled you. The thought of recurrence can be hard to come to terms with but it helps me to think that I have followed all that my doctors recommended and I live a healthy lifestyle to further reduce my risk. I don't think you need to think you will regret following the specialists advice. That is the best we can do. Take care. Deanne xxx