I have had recurring resistant ringworm for the last 3 years. my doc treats it with 10 days oral terbinafine which works but then it comes back after 2 months. i don't have any immune problems. now it's back & widespread. why is this happening?

One of the main reasons good treatments fail to cure a problem is the diagnosis is wrong. I would see a dermatologist and see if this is one of several issues that can mimic ringworm.

I would suggest to repeat a 2 week course of terbinafine and you have to follow preventive measures thoroughly: avoid contact with sources infested with ringworm or treat infested family members or pets, do not sharing bathing accessories, dry yourself completely after bathing, avoid tight clothing, change socks and underwear daily, wash and dry thoroughly all clothes & bedding accessories once

I might also add an antifungal wash/shampoo such as ketoconazole shampoo 2% 1-2 times per week on the scalp , body and exts following the 2 week course of oral terbenafine. Should there still be a recurrence, a medical workup should be done