persistence in dua (supplication)

قَالَ رَبِّ إِنِّى وَهَنَ ٱلْعَظْمُ مِنِّى وَٱشْتَعَلَ ٱلرَّأْسُ شَيْبًا وَلَمْ أَكُنۢ بِدُعَآئِكَ رَبِّ شَقِيًّا

He said, "My Lord, indeed my bones have weakened, and my head has filled with white, and never have I been in my supplication to You, my Lord, unhappy [i.e., disappointed].