grind down

verb2

1. •
   To make something hard smaller and smoother by gradually rubbing off tiny bits; to wear down by grinding action.

   • The old dog's teeth had been ground down by constant use.
2. •
   To weaken someone's morale over a long period.

   • Djokovic broke twice in each of the opening two sets, dismantling 6ft 8in Anderson's service game and grinding him down in the rallies. — 2018