What is the ruling on interrupting a supererogatory prayer upon losing humility (khushu'), and is there a fear that the prayer's supplication will be lost due to the absence of humility in it?

The absence of khushu' (devotional humility) in prayer, whether obligatory or supererogatory, does not justify abandoning it or breaking it off. A Muslim must strive to attain khushu' in prayer, and remember the greatness of Allah and that this might be their last prayer. Khushu' is not a condition for the validity of prayer according to the majority of scholars, and prayer is not invalidated by its absence. One who lacks khushu' is not required to repeat the prayer, but its reward is diminished. The aforementioned hadith (definition) encourages maintaining prayer with all its pillars and khushu', and warns against neglecting it. It does not mean that the solution for lack of khushu' is to abandon prayer, but rather to strive to attain it and seek Allah's help.