Complex sentence: He has not yet told Mafuyu all of her forgotten past memories between the two of them, one in particular about an accident she was involved in right before he moved which was the reason she lost some of her memories about him in the first place.
Split into simpler sentences: He has not yet told Mafuyu all of her forgotten past memories between the two of them, one in particular about an accident she was involved in right before he moved. Which was the reason she lost some of her memories about him in the first place.