I've been on tamoxifen for 6 weeks after finishing radiotherapy in April for breast cancer and developed vertigo last saturday - I can really sympathise with how you're feeling... 

The breast care nurses have advised me to persevere and I've got a follow up appointment in August at the hospital.  I looked online at self help.  It shows how to do a home Epley procedure which helps restore inner ear crystals to the correct position, so may try it.  Also suggests trying ginger root tea (slithers of root ginger in boiling water)