Hi, I'm a doctor. Paroxetine and fluoxetine should not be used with tamoxifen as they make the tamoxifen less effective. There are other alternatives to paroxetine in the same family which do not interact anything like as much, such as sertraline and citalopram. Suggest you talk to your GP about it.
I'm on sertraline and will be on tamoxifen after chemo. Just to warn you, if changing from paroxetine, it can take a long time to come off as you may need to reduce very slowly as side-effects from reduction can be nasty (paroxetine is particulaly bad for this I'm afraid)
Also the anxiety will improve a bit once treatment is underway, the bit at the beginning when you're waiting all the time makes everyone anxious, but it does get better.