My left eye droops badly when I get tired, but is fine most other times. Is it...
Your observation is not unusual.
If you are examined by an ophthalmologist you probably will have PTOSIS of the upper lid. By the end of the day the ptosis gets worse because the muscle gets tired.
Correction is possible with a "Ptosis Procedure"
Have a complete ophthalmologic exam. Then proceed with a consultation with a Board Certified Plastic Surgeon (or Board Certified Opthalmologic Plastic Surgeon) for repair.
You need an evaluation to make sure there are no medical conditions causing this. An ophthalmologist is a good start. After an evaluation and diagnosis, the appropriate treatment can be recommended.
This sounds as though you have a problem with the non-voluntary muscle called ptosis. Correction of this is relatively simple, but takes significant finesse. You need an evaluation by a plastic or ophthalmic plastic surgeon who is experienced with eyelid ptosis correction and does it regularly.