Is the prayer of one suffering from a nervous breakdown valid if he sits while performing it, with frequent lapses in concentration and forgetfulness, and an inability to recall what has preceded of it?

Standing in prayer is a pillar without which the prayer is not valid. Whoever is unable to stand may pray while sitting, based on the Prophet's (PBUH) saying: "Pray standing; if you are unable, then sitting; if you are unable, then on your side." If you are able to stand sometimes, you must do so, based on the Almighty's saying: "So fear Allah as much as you are able," and the Prophet's (PBUH) saying: "If I command you to do something, then do of it as much as you are able."

Frequent misgivings (waswas) and absentmindedness do not invalidate the prayer, but they diminish its reward in proportion to the mind's preoccupation during it. The worshipper should repel these thoughts by pondering over what he recites or hears, and by being mindful of Allah's greatness and His watchful presence, and by seeking refuge with Allah from the accursed Satan. Prayer in this state is valid, and one fulfills their obligation, even if the reward is diminished.