As for forgetting one verse in the recitation (other than Al-Fatihah) during Prayer, Sheikh Mohamed El-Moctar El-Shinqiti, Director of the Islamic Center of South Plains, Lubbock, Texas, states: if one forgot to read one verse during Prayer (other than Al-Fatihah which is a pillar in the Prayer) and someone corrected the person or one remembers while they are still reading, one can simply come back and correct the mistake and resume the recitation. If one has found out that they forgot a verse after they made ruku` or after Prayer, there is nothing wrong at all with the prayer and do not have to break that prayer or start again as one did so out of forgetfulness. The Prophet (peace and blessings be upon him) is reported to have said, “Allah will not hold anyone of this nation responsible for what is done in error, forgetfulness, or under coercion”(Al-Bukhari).
However, If one forgot a verse from al-Fatihah and remembered this while they are still in the same Rak`ah, then they have to be back to read al-Fatihah correctly and go ahead with the other pillars in a normal way. If they remembered after they finished that rak`ah in which they did the mistake, then consider that rak`ah as invalid and add a rak`ah to replace it. If one remembers after finishing the prayer, then they are to repeat the prayer again. However, all these when you are sure or pretty sure that you did a mistake; so if you are only doubting, then your prayer is ok as doubts don’t remove certainty.