What Causes Dryness And Brittleness Of Nails?

Hi I am on tamoxifen from breast cancer since 2012. I just had radiation.Lump removed only, I am doing great, but my nails sad situation. Dry, bad cuticles, not growing, yes I pick at them too. Only thumb has hard vertical ridges. I have not polished them in about 2 years I just file them so very short.A few months months back I started taking Bonita for hair, skin and nails fro health food store, does not help really what can I do? I sure miss my nails!!!!!! Jolene