Town go into the first leg at the John Smith's Stadium on Sunday on the back of successive defeats against a side which completed a league double over them this season.
But Whitehead said anything that has gone before, in the regular campaign, counts for nothing.
He said: "The season is over and the play-offs are a new competition. Form doesn't really count for anything in my view.
"We will go into the game with some confidence and we want to obviously perform well in that first leg.
"I think everyone is aware that these are huge games, however we'll stay relaxed, focused and stay together in the dressing room.
"It's all about which team is best on the day and the past counts for nothing."
On the Cardiff City game, Whitehead added: "It has been a difficult few weeks since that Wolves game where we secured the play-off place.
"I am glad to get the Cardiff game out of the way. Our focus now has to be fully on Sheffield Wednesday."