Can a left choroidal fissure cyst of 12mm affect vision in left eye. blurry and double vision on and off.

Not likely: cysts of the choroid fissure or plexus are a common finding and totally asymptomatic. If you have optic neuritis this is something a neurologist and ophthalmologist need to be following. It can be a symptom of possible multiple sclerosis. It can be from other causes. Did you have a MRI with and without contrast? If they are looking for MS they will do additional images - to find lesions