How can a patient with OCD be reassured about the soundness of their prayer and acts of worship after a period of negligence and a feeling of shortcoming?

The cure for obsessive whispers (waswas) is through the remembrance of Allah, holding fast to Him, relying upon Him, and maintaining regular recitation of the prescribed remembrances (adhkar). These form an impenetrable fortress against Satan. One must also learn the guidance of the Messenger of Allah, peace be upon him, regarding purification (taharah), prayer (salat), fasting (siyam), and other matters. If one combines reliance upon Allah with insight into the guidance of the Messenger, then they will not heed the whispers of Satan. It is not permissible for a person to abandon worship or become lazy in prayer under the pretext of waswas; rather, they must strive against themselves and against Satan.