What must I do after it became clear to me that my praying while sitting for years, believing it permissible though disliked, was incorrect and that standing is a pillar of prayer?

Standing in prayer is a pillar that is not waived as long as one is able to do so. Therefore, whoever prays sitting while being able to stand, their prayer is invalid.

The questioner must make up the prayers she performed sitting while she was able to stand. If she can enumerate them, she should make them all up; otherwise, she should make up the number of prayers she predominantly believes to be the correct amount.

This is the view of the majority of scholars, whether the worshipper knew that standing is a pillar or was ignorant of it.

Ibn Taymiyyah held the view that a person ignorant of the ruling regarding standing is not obligated to repeat past prayers, but rather should repeat the current prayer and commit to standing in the future, citing the Hadith of the one who prayed badly as evidence.

The view of the majority of scholars is the more correct one.