What is the reason for the difficulty of supererogatory prayers, especially those performed after the 'Isha prayer?

A Muslim's feeling of burden from supererogatory prayers does not necessarily indicate demonic possession. A servant's faith increases and decreases, and lethargy can be due to sins, exaggeration in worship, or a physical or psychological illness, or possession. We advise striving against the self to perform the regular sunnah (definition) prayers without exaggeration, and to beware of abandoning the sunnah.