What happened to Augustus leg in The Fault in Our Stars?

That question can be interpreted in many different ways- and the answer changes as the novel progresses. Physically he was fighting with his disease- Cancer. But mentally, Augustus Waters was battling with the idea of his death (he always believed that in life, he had to do something big to make it worth anything) and his identity. Because of his young age (seventeen) he had yet to figure out his life and as the novel progressed we see him detiorating slowly until the scene at the gas station where he loses his hold with everything for a moment with Hazel Grace. Until then he had played at being a hero, but from the moment his cancer returned, we saw his downfall, slowly.