Kaneki lost his memory after Kaneki lose his memories in Tokyo Ghoul fight against Arima (CCG) where he pierced both his eyes. Thanks to Kaneki’s regenerative abilities, his brain rebuilded itself to allow him to survive, however his memories couldn’t be “regenerated” and were, unfortunately, lost.
In the end, he doesn’t give up, he fights through the struggles, and is strong where it matters most. So, throughout the story, yes, Kaneki is weak. He gives up and loses himself, again and again and again.