Derby dominated the match at Wembley Stadium on Saturday, but Bobby Zamora struck in the final minute to seal 10-man QPR's promotion to the Premier League.
"We have a habit of doing it the hard way," Barton told BBC Sport.
"I knew we would do it but I feel sorry for Derby, they played fantastic today and they deserved to win. That could have been us."
QPR return to the top tier of English football having spent just one season in the Championship.