QPR were reduced to ten men in the 21st minute when Bobby Zamora was sent off for a high challenge on Jordi Gomez, but it looked like they would run out winners after Loic Remy's fantastic goal five minutes from time.
However, Shaun Maloney's last-gasp equaliser left QPR seven points from safety with six games of the season remaining, and Redknapp was stunned by the late setback.
Asked if he had encountered a tougher result to take as QPR manager, Redknapp replied: "All season? I don't think I've had a tougher result to take in my football career, never mind all season."
He told Sky Sports: "We had worked so hard with ten men all game and I felt we deserved a goal. It was a fantastic goal and then we gave two silly free-kicks away on the edge of the box and the second one allowed them to score with the last kick of the game.
"It couldn't have been more cruel. We didn't deserve that. We deserved to win today with ten men.
"The players were fantastic. Every one of them worked their socks off and you can't ask for more than that. The ability they played with was absolutely top class."
Redknapp added: "We have shown spirit every week since I've been here. You can't ask for more. The players I've brought in have done great. Remy's scored goals, Andros Townsend was magnificent today, Jermaine Jenas worked his socks off in the middle of the park and Chris Samba put in a performance.
"We are a much improved team, but unfortunately it is hard when you are playing catch-up all year.
"We have got to keep believing but you need a bit of luck sometimes and we didn't get any today.
"To be fair to Wigan, and I am a big fan of the manager, but they had all the luck in the world at Newcastle the other day, and today I am sure they had almost given up with ten seconds to go."
Redknapp had no complaints with the decision to send off Zamora, and revealed the striker was disappointed with himself.
"I think it looked like a red card. Bobby's not a malicious player but it was a clumsy challenge," Redknapp said.
"His foot was high and it was a reckless challenge. It is not like Bobby to do things like that, he is normally very level-headed.
"Nobody is more sorry than Bobby.
"We would have beaten them today with 11 men, I am sure of that, but even with ten we should have won.
"Then you are down to ten men against a team that likes to pass the ball, but I thought we always looked more threatening."