"It is, very, very hard to take," Barrett said.
"It's probably been a pretty common element in all our games. There's just a 15 minute period where we just capitulate and we can't handle any adversity. That's what happened again.
"That was a massive game in terms of our season. Now it's going to be real hard (to make the finals).
Barrett also conceded the match-fixing allegations surrounding his club were affecting his players." We've got some young players in there who aren't used to dealing with the kind of adversity we're in at the moment. I get that and understand that," he said.
"The way they handled themselves off the field is a credit to the players. I'll never question their effort and their commitment to what we're doing.
"That's what upsets me the most, we put ourselves in a position to win on the back of how hard they've worked and to throw it away as easily as what we have. It was the same in Canberra last week."
Two first half tries to Walker, another to Tom Trbojevic and Jorge Taufua's 46th minute touchdown to put Manly ahead 24-4 looked set to give them their fifth win of the year.But the Panthers came storming home.
After a lone Tyrone Peachey try in the first half, tries to Josh Mansour in the 49th minute, Nathan Cleary, 55th minute, Lelani Latu 67th minute, and Peter Wallace in the 71st minute put the visitors ahead 26-24.
Dallin Watene Zelezniak put Penrith further ahead in the 75th minute before a Matt Moylan field goal two minutes from time sealed the incredible win.Panthers coach Anthony Griffin said he never lost faith in his team.
"I always thought if we could get some ball, we were still a chance," he said.
"That game could have spiralled out of our control, but we hung in there."There is a lot of belief in this side."In the absence of Jamie Soward, Nathan Cleary and Bryce Cartwright led the Panthers well in the halves.
Moylan had an inconsistent game as he, like Walker, seeks to hold his place in the Blues side for Origin II.
- AAP
