You did not return to work fine. You returned with Post Concussion Syndrome. You had been living a low stress life and your symptoms were manageable. Now, with work stresses, your symptoms are no longer manageable. It sounds like you tolerated the stresses last summer so your PCS is a disability. Ask to speak to Human Resources/Personnel or an ADA specialist (Americans with Disabilities Act). Explain that you love your job and want to keep it but the heat and rough roads are too much for your PCS brain.

Is the job directly with the government or are you employed by an outside contractor ? Direct employment should give you more rights.