Does the prayer of one who has said the taslim become invalid if he is almost certain that he did not complete the correct formula for the Abrahamic prayer in the final tashahhud, and should he repeat the prayer, given that he experiences obsessive-compulsive disorder regarding the intention for prayer, which lasts for an hour?

The cure for obsessive whispers (waswas) is to ignore them and not pay them any attention. When you stand for prayer, make your intention in your heart, then immediately begin with the takbirat al-ihram without delay. Proceed with your prayer and do not repeat anything. The prayer you asked about is valid. The obligatory amount of salat (blessings) upon the Prophet, peace be upon him, in the tashahhud is to say: "Allahumma salli 'ala Muhammad" (O Allah, bless Muhammad). Anything more than that is sunnah (recommended). Reciting salat upon the Prophet, peace be upon him, in the final tashahhud comes after the phrases of the tashahhud, which is a pillar without which the prayer is not valid. However, if it was omitted previously, it is hoped that you are not required to repeat the prayer, because many scholars believe that the final tashahhud is not obligatory, and that whoever omits something from the conditions or pillars of prayer out of ignorance is not required to repeat it. Therefore, in the future, adhere to the full phrases of the tashahhud in the final tashahhud, then the salat upon the Prophet, peace be upon him, then the du'a (supplication), then the taslim (salutation of peace).