I don't have any experience of Tamoxifen, but I do have experience of another oestrogen blocker called Exemestane, which I've been taking for over two years after breast cancer. At first I was prescribed Letrozole, which didn't suit me, as I felt very nauseous.
The only side effect I've had with Exemestane is hair thinning and a slight loss of bone density. There's not much I can do about the hair thinning. I've now had two DEXA scans, which I expect you'll also have and have been diagnosed with osteopenia, for which I've been advised to ensure adequate intake of calcium and Vitamin D and to do regular weight bearing exercises (mainly walking).
Research shows that drugs like Tamoxifen do work in helping to prevent breast cancer from returning. Quite honestly, I'm more scared of that happening than any side effects from the drug. At least you can stop taking the drug if you have severe side effects and can try another one, which is much better than dealing with a recurrence of cancer.