Am I obliged to memorize what I have forgotten of the Qur'an after my return to Allah?

All praise is due to Allah, and peace and blessings be upon the Messenger of Allah, and upon his family and companions. To proceed:

Indeed, abandoning prayer is one of the gravest sins, and it may even constitute disbelief that expels one from the fold of Islam. Therefore, one must be diligent in observing it. As for forgetting the Quran and neglecting it, this is a sin and a calamity, and it is a consequence of neglecting prayer. Hence, one should strive to revise it once again; perhaps that will atone for forgetting it and for the neglect in preserving it.

And Allah knows best.