The Hoops took the lead five minutes from time through Loic Remy and looked to have sealed a priceless victory, until Shaun Maloney equalised with the final kick of the game to secure a 1-1 stalemate at Loftus Road.
QPR are now seven points from safety in 19th with just six games to go, and Redknapp believes that he has never felt so low during his career.
"It feels like the worst defeat of my life," Redknapp told talkSPORT. "I don't know how I'm going to lift myself. I'd be a liar if I said I could lift myself tonight. I'm low. Tonight is probably as low as I've ever felt.
"I thought it was the cruellest blow I've ever suffered since I've been in football. Their goal wasn't deserved. We should have won the game. [They scored] in the last minute of four minutes of injury time. We thoroughly deserved the win today. The work rate, the effort we play with I thought was deserving of three points.
"Three points, if we'd have got them today, would have given us a fantastic opportunity to stay up. It was a massive disappointment. We needed that win today and with 10 men that would have given us such a lift."
Bobby Zamora was sent off in the 21st minute of the match for Rangers.