on page 66

"the thousand times that he had proved it meant nothing. Now he was proving it again. Each time was a new time and he never thought about the past when he was doing it." saying the past is done and over, and he has to focus on his stuff now.

This is just an expansion on your thought...

"The thousand times that he had proved it" = Hemingway's past literature works achieving critical fame

"meant nothing" = It is what you can write now that matters. The past is the past. Hemingway could possibly be feeling that every time he writes something, it was all for nothing, because eventually, wouldn't it be in the past? The feeling of having to constantly prove to others of his writing could to dragging him instead of moving him forward. These two words made be think of Hemingway possibly think that "my writing means nothing, therefore me life means nothing."

"he was proving it again" = Hemingway trying to prove that his writing skills are not in the past. He is still that great writer.

"each time was a new time" = constantly reinventing his writing in order to make it better

"he never thought about the past" = Perhaps Hemingway was repressing unpleasant events that happened in the past as a result of his writing? Hemingway may have thought that if he continued to think of the past, it would only bring him down.

it could also be talking about his writing career. Like in his eyes all his other books were failures or something.