Thomas concedes the team were under-prepared for their win-or-bust clash with Fiji and were "enveloped" by fear before facing Australia.
He has also described Gareth Jenkins' decision to leave him out of the first game as "crushing" but insists blame must be shared for the failure to reach the quarter-finals.
Thomas has given his thoughts in his autobiography 'Alfie: The Gareth Thomas Story', which is being serialised by the Western Mail.
Wales were left needing to beat Fiji in their final pool game after losing to Australia but lost 38-34 in Nantes.
Thomas wrote: "I was totally confident that, if we played to our strengths, we would beat Fiji comfortably.
"All week, I tried to instil into the boys the need to be confident about the game. Yes, Fiji were dangerous, and, yes, they had their strengths, but I didn't want us to focus on what they might do to us.
"Instead, I wanted us to concentrate on what we could do to them. 'Fiji are a good team, but if we perform, we can blow them out of the water', I told the players.
"We hadn't really watched Fiji that much, which only meant the shock we had on the day was the more acute.
"I don't know if Fiji will ever perform that well when it really counts again. All I know is we caught them on one hell of a day."
Thomas had also been confident ahead of the clash with Australia at the Millennium Stadium, but Wales were beaten 32-20 after slumping 25-3 behind in the first half.
"We were confident and buzzing going into Australia game," he wrote. "I didn't know why, but as we stood in our positions before kick-off, a sense of fear suddenly enveloped us."
The tournament began for Thomas on a low note as he was left out of the starting XV to face Canada in the opening game by coach Jenkins.
He said: "I was chuffed to bits to get the armband back, but when it came to the big moment I was hit by what was at the time a crushing decision by Gareth Jenkins.
"I was devastated, absolutely gutted, and I couldn't understand it. I wondered if he even thought that I was part of his best XV.
"I think the world of Peely [Dwayne Peel] and wanted only the best for him, but I'm only human, and I couldn't get away from the view that someone else was doing my job."
Yet despite Jenkins' decision, Thomas believes it would be harsh to blame the coach solely for the team's shortcomings.
"Gareth Jenkins bore the brunt of it, losing his job almost immediately after we had been eliminated," he said.
"But let's be clear about this. We were all to blame - players as well as management. Unfortunately, in professional sport, the buck always stops with the coach."