Is the prayer of a person afflicted with OCD invalid if he refuses the idea of repeating verses in loud prayer and says "Astaghfirullah" (I ask forgiveness from Allah) and performs the prostration of forgetfulness (sujood as-sahw) after the salaam, and what should be done to rectify it?

The ideal solution for someone afflicted with waswasah (whispers of the Devil) is to ignore the waswasah, seek refuge in Allah from the accursed Shaitan, and continue the prayer without performing the prostration of forgetfulness (sujud al-sahw); because it is waswasah and not an act of forgetfulness.