Welcome @ripley, It must be really hard not knowing what's causing the left sided headaches and having them go away for awhile and then return. There are a couple of discussions that sound like they could be related to your symptoms.

-- Temporal biopsies to diagnose GCA: What's your experience?: https://connect.mayoclinic.org/discussion/gca-biopsies/
-- Occipital Neuralgia: https://connect.mayoclinic.org/discussion/occipital-neuralgia/

I'm not sure if this answers your question can it really be temporal arteritis going on for 2-1/2 years but it sounds like a possibility. "The most common symptom of temporal arteritis is a throbbing, continuous headache on one or both sides of the forehead. Other symptoms may include: Fatigue. Fever." --- Temporal Arteritis: Treatment, Symptoms & Diagnosis: https://my.clevelandclinic.org/health/diseases/15674-temporal-arteritis

@zaa and @tsc may have some thoughts or suggestions to share with you. Have you thought about seeking a second opinion or maybe talking with a rheumatologist?