How did Douglas make sure that he conquered the old terror?

Even after the swimming training was over, Douglas wasn't confident about his swimming or that he had overcome the fear. He was determined to completely get rid of it forever.  He swam alone in the pool. He went to Lake Wentworth to dive. There, he tried every possible stroke he had learnt. He fought back the tiny vestiges of terror that gripped him in middle of the lake. Finally, in his diving expedition in the Warm Lake, he realised that he had truly conquered his old terror.