Are my previous prayers unacceptable, and what is their expiation, knowing that I was unaware that discharge after masturbation or upon waking necessitates ghusl, and I did not know the meaning of الاحتلام (nocturnal emission), what entails from it, or how it occurs?

Ghusl (ritual bathing) is not necessitated merely by a wet dream or by masturbation; rather, it becomes obligatory upon the emission of semen. If you are certain of the emission of semen which necessitates ghusl, then you must perform ghusl for your prayer to be valid. If you are doubtful about the nature of the discharge, you have the option to choose its ruling. A wet dream is a dream of intercourse, and not every wetness thereafter necessitates ghusl; rather, certainty of the emission of that which necessitates it is required. It is advised to disregard obsessive whispers and not to dwell on them.