Town have taken seven points from their last three matches and currently lie seven points off sixth place with three games left.
They head to Oldham on Saturday with Barker knowing only a victory will be enough to leave them with any chance of staying in the hunt.
"We probably need all the sides from Swindon down to us to lose one of their last two games for us to have a chance so there's no point in us leaving anything behind on Saturday," said Barker.
"I'm going to be positive and look forward rather than back at some of the points we've dropped.
"I asked the players to be in with a shout nearer the end of the season and that's where we are. We need other teams to drop points but we're not a millions miles away."