What should a person do who frequently interrupts prayer, experiences depression, and feels as though they have a mental illness?

Abandoning prayer is a grave sin that can lead one to disbelief (kufr). The Companions of the Prophet, peace be upon him, did not consider any act, when abandoned, to be disbelief except prayer. Prayer is obligatory, and satanic whispers do not exempt one from performing it; rather, prayer brings healing, psychological comfort, and tranquility. One must hasten to repent from having ceased prayer.