When. In grief and prolonged anxiety — the kind that does not lift by evening. It is long, and being long is part of how it works: it is not a phrase to fire off, it is something to be recited slowly by a person who has time because they are not sleeping.
The words. Nasiyati biyadik — "my forelock is in Your hand". The forelock is the hair at the front of the head, and taking hold of it is how an animal is led or a captive is controlled. The image is of being held at the point where direction is decided. A person in anxiety feels precisely the opposite: that nothing is holding anything.
Then two clauses in sequence: madin fiyya hukmuk — Your command over me is carried through — and 'adlun fiyya qada'uk — Your decree concerning me is just. The first admits that what happens will happen. The second is the harder one, and it is said before any relief has arrived.
What is actually requested. After all the acknowledgement, the request is not that the difficulty be removed. It is an taj'alal-Qur'ana rabi'a qalbi — that the Qur'an be made the spring of my heart. Rabi' is springtime, when the rain has come and the ground turns; it describes what happens to dry land, which is what a grieving person is. The request is for the source of relief, not for the conditions to change.
Its opening. The three descriptions — Your servant, son of Your servant, son of Your maidservant — place the speaker in a line of servitude that predates his particular trouble.