Congenital Ptosis: Given the long-standing history of ptosis since childhood (20 years), congenital ptosis is a strong consideration. The fact that it has worsened over the past 7 years could be due to progressive stretching of the levator palpebrae superioris muscle or other anatomical changes over time.

Aponeurotic Ptosis: This condition involves disinsertion or stretching of the levator aponeurosis, which could be exacerbated by the presence of a foreign body (metal corpus alienum) in the eye, leading to inflammation or scarring that affects the eyelid's lifting mechanism.
Traumatic Ptosis: Although the patient mentions the presence of a metal foreign body a few times, if these incidents were significant enough, they could have caused trauma leading to ptosis, either through direct injury to the levator muscle or aponeurosis or through scarring.

Third Cranial Nerve Palsy: While less likely given the chronic nature of the symptoms, a third cranial nerve palsy could cause ptosis. It's crucial to rule out this condition, especially if there are other symptoms such as diplopia or pupillary dysfunction, as it could be a sign of a more serious underlying condition like an aneurysm.
Myasthenia Gravis: This autoimmune disorder can cause fluctuating ptosis that worsens with fatigue. Although it might not be the first consideration given the long history, it's a critical diagnosis not to miss due to its systemic implications and the availability of treatment.

Chronic Progressive External Ophthalmoplegia (CPEO): A mitochondrial disorder that can cause progressive ptosis and external ophthalmoplegia. It's rare but should be considered in cases of long-standing, progressive ptosis without other clear causes.
Levator Muscle Biopsy-Proven Mitochondrial Myopathy: Similar to CPEO, this would be a rare cause of ptosis but is worth mentioning due to its progressive nature and the need for specific diagnostic testing for confirmation.