Aside from the 'no going back' aspect,
I'm not sure what you mean by this?
The recommendation is to take Tamoxifen for 10 years but nobody forces it down your throat. You can choose to stop taking it at any time.
Sorry to say that if you are already perimenopausal you may not get to 10 years because usually when it is confirmed that you are post menopausal you will be switched to an aromatase inhibitor - probably Arimidex or Aromasin.
Don't worry about side effects before they happen - who knows you could be one of the lucky ones who barely notices it.
Good luck.