What is the ruling on someone who today realized their mistake in reciting Al-Fatihah after believing it to be correct, and has begun to rectify it?

An error in reciting Surat Al-Fatihah does not invalidate the prayer if it does not change the meaning. However, if it changes the meaning, then it invalidates it. An unintentional error in Al-Fatihah—even if it is a clear grammatical mistake (laḥn jalī)—does not invalidate the prayer according to some scholars, such as the Malikis. It is permissible to follow this opinion, and therefore, there is no need to repeat past prayers in which this error occurred.