What is the ruling on the prayer of someone who inadvertently added the word "ya" (O) in the final Tashahhud, and should he repeat the Dhuhr prayer or the Jumu'ah (Friday) prayer? And how does one differentiate between an error that invalidates the prayer and a mere whisper (waswasa)?

Your prayer is valid and does not need to be repeated, as your addition of the vocative particle "Ya" was a mistake due to forgetfulness, and this does not invalidate the prayer. Mere doubt is not considered obsessive whispering (waswasa) until it becomes frequent. We ask Allah to cure you from waswasa, and we advise you to supplicate, distract yourself from it, and consult a trusted psychiatrist.