Is it obligatory to repeat the prayer if it was performed in the opposite direction of the Qibla due to forgetfulness, based on a mistaken assumption?

If the situation is as described, and you were praying in the wrong direction of the Qibla, then you must repeat the prayers you performed not facing the Qibla, especially if you prayed without making an effort to determine the Qibla or asking someone to guide you to its direction. You can refer to related fatwas regarding how to make up many missed prayers.