The Wallabies were eliminated by eventual runners-up England at the quarter-final stage and the pain still lingers for Mitchell and his team-mates.
Speaking on Tuesday in Perth, Mitchell said: "A lot of guys had been working for so long with the main goal to be world champions.
"For that to end a lot earlier than we thought it could have was very disappointing. It was hard to deal with and hard to take. It all happened very suddenly in those 80 minutes, and the next day we said our goodbyes and departed from there."
But Mitchell is using the hurt as a spur and promises to be one of the competition's hottest prospects after his personal success at the World Cup, when he came of age by scoring seven tries in five matches.
"The result was bitterly disappointing, but we've had a few weeks now to try to forget about it and put it past us," he added.
"We just need to focus on what's ahead with the Western Force.
"I've had a pretty good holiday and am now looking forward to getting back into it. I've got a lot of confidence coming out of the World Cup, and if I can keep riding that and keep that consistency in the Super 14s hopefully I can keep going forward."
At the Force, Mitchell will team up with fellow Wallabies Nathan Sharpe, Cameron Shepherd and Matt Giteau.
Scott Staniforth should also be fit after coming through a shoulder operation last week, and Mitchell is looking forward to teaming up with his World Cup colleagues on the domestic stage.
"The World Cup is the biggest campaign you can go through, and quite a few experiences you can take on board. Losing in the quarter-finals was a massive disappointment, but if we can take anything from that and use it in our game with the Force, hopefully we benefit from it," he said.