43, left eye feels heavy vs. the right - why is this happening? (Photo)

Seems like you have ptosis on one side and to compensate your frontal muscle is more active on left side and your eyebrow is higher. It is not possible to correctly diagnose the problem with only one photo, I'd recommend a consultation with a doctor.

You have some asymmetry of your orbits, and I believe your left orbital socket sits higher in your skull and may have a different shape compared to the right side.
Regardless you appear to have ptosis of both your upper eyelids and most likely this is causing some blocked peripheral vision.
Consider consulting with oculoplastic surgeons in your community or any type of plastic surgeon.
For eyelid issues, oculoplastic surgeons may have some advantage.
I do believe that skeletal structure is contributing significantly, though this probably does not explain your upper eyelid ptosis.
If your upper eyelids are blocking your peripheral vision, then medical insurance may cover surgery.
Best,
Mats Hagstrom MD