The refrain is unbearable because it is polite. He never accuses her of having forgotten. He offers both possibilities every time, with exactly equal weight, and after eleven couplets her memory has become a room he is not allowed into. He can inventory everything he still has. He has no way of finding out whether she kept any of it.
Once, and only once, he drops the hesitation. In the second couplet he says it outright: he remembers everything, zara zara, every smallest piece. For that one moment the remembering is his rather than hers. Then the pattern resumes, and the refrain sounds different now, because we have heard what one answer looks like and she is still the only one who has not given it.
What keeps this ghazal alive is the sort of thing he chooses to remember. No grand declarations anywhere. Sulking over small matters. Talking in signs because other people were in the room. Complaints about being scolded by relatives. The no, no offered at every moment out of habit rather than refusal. These are the details a love affair actually runs on, and the poem has kept every one of them. There is nothing in the list anyone could dispute.
The last couplet puts his name to work. Momin is his pen-name and, in plain Urdu, just the word for a believer, someone who keeps faith. So the line does two jobs at once: I am the man you once called faithful, and I am still the one keeping it. He signs the poem with the very quality it is about. Then the refrain returns one last time and folds the whole thing back into the question.
Begum Akhtar's is the reading the ghazal is usually remembered by. Farida Khanum takes it slower and lets the refrain reach its full weight, which is not a comfortable thing to sit through eleven times.
How it rhymes
Every second line closes on the same word — تمہیں یاد ہو کہ نہ یاد ہو
, tumhen yaad ho ki na yaad ho. That repetition is the radif.
The rhyme falling just before it is the qafiya: The -a sound just before the refrain: qarar tha, haal par, ruthna, barmala, aqriba, bhulna, ashna, kya, bala, ada, mubtala.
Hearing the two land together at the close of every sher is most of what makes this form sound the
way it does.