15 day headache, almost constant blurred vision in one eye. ER 3x. Neurologist, Opthamologist. Who diagnoses giant cell arteritis? Who should I see?

Headache ? G C arter. Usually G C arteritis can be included/excluded by a sedan rate ( blood test) with your symptoms I would start with your primary care doctor or a neurologist. This may require addition testing such as an mri. While I can't say you have it but I would exclude optic neuritis as a diagnosis. Good luck. .
Tests then biopsy. Has an ESR test been done (erythrocyte sedimentation rate)? If this is normal giant cell arteritis is reliably excluded. If it is significantly elevated and Dr agrees symptoms are suggestive, a biopsy is usually be next step & is really the ONLY way to be SURE about this diagnosis. Its easy on the patient, done in Dr. office, takes about 20 min, local anesthesia, very low risks. Hope this helps. .