From the looks of things, he's a lot happier with this new coach. And he did get to the finals of a grand slam pretty quick with this guy, so it's probably in his best interests to keep him. Also, would gilbert want to be rehired after being unceremoniously dumped? I probably wouldnt want to be!
Yes it was a shame he was physically unfit at the end of this year, he had a good one overall. His first injury was his hip when he fell during that Wimbledon final, which must have been an even bigger kick in the teeth after the final score.
His mental strength is certainly a factor. He often gets frustrated and lets it get to him; something Delpo has also struggled with. But Roddick is a older and more experienced, so he shouldn't really let it take control!