Is it obligatory to repeat a prayer in which there was no khushu' (devotion) and concentration? And what is the advice when that recurs?

If a Muslim grieves over their shortcomings in obedience, this is a sign of the sincerity of their faith. The self-reproaching soul that blames its owner for shortcomings, despite effort being expended, is a soul not to be blamed. A Muslim should strive to complete acts of worship and beware of excessive self-reproach or leniency. The remedy lies in being diligent about what strengthens obedience and weakens disobedience, and in turning to Allah through righteous deeds and remembrance. Focusing in prayer as if it were the last, along with conquering desires and whims, are among the causes of presence of heart. Hearts are of three types: a heart devoid of good, in which Satan finds rest; a heart enlightened by faith but overwhelmed by desires; and a heart filled with faith and illuminated by it. This last heart is guarded from Satan. Prayer is not to be repeated due to a lack of khushu' (devotion); rather, what is required is to strive to perfect and improve it, and the matter requires perseverance and patience.