Do prayers from past years, in which I intentionally omitted reciting a surah after Al-Fatihah based on a previous fatwa, become invalid? Must I repeat them, or can I continue to suffice with Al-Fatihah as a concession due to the recurring problem of forgetting to recite Al-Fatihah?

The madhhab of the majority of scholars is that reciting a surah after Al-Fatihah is a Sunnah, and its omission, whether intentional or by oversight, does not invalidate the prayer. Nor does it necessitate the prostration of forgetfulness (sujood as-sahw). Therefore, past prayers in which the surah after Al-Fatihah was omitted are valid and do not need to be repeated. As for the Maliki madhhab, it holds that sujood as-sahw is obligatory for one who omits the surah by oversight, and that the prayer becomes invalid if it is omitted intentionally. However, the more authoritative (rajih) opinion is that of the majority of scholars. It is recommended to strive to achieve full presence of heart in prayer to attain the complete reward.