Manager Grant McCann has apologised to the fans for Peterborough United's 4-1 home defeat by fellow League One play-off hopefuls Southend United.
Victory lifted the Shrimpers up to fifth, while Posh have only won six of their 17 home league games this term.
"If you don't believe you're going to get there [to the play-offs] you never will. My belief won't stop until it's mathematically impossible," he said.
"We're five points off the play-offs. It's not a lot with 13 games to go."
He continued to BBC Radio Cambridgeshire: "I know we've just been thrashed at home, but we need to continue to support each other. All I can do is apologise to the fans after the performance.
"I always take the blame when we lose and I'll do it again now. It's down to me why the team lost. What we've got to do now is stand up like men, puff our chests out, come back on Saturday and produce a win."
Two goals from Marc-Antoine Fortune set the Shrimpers on their way to victory at the ABAX Stadium, while Tom Nichols scored the only goal for ninth-placed Posh.
However, McCann has received positive news regarding Gwion Edwards' knee injury, picked up in Saturday's defeat by Walsall.
"It'll be seven or eight weeks instead of seven or eight months. We feared the worst to be honest, but we got the good news it's not too serious.
"He's been top drawer this season, he's got a great attitude and he's a tough little cookie."