The 2-0 loss to the Addicks on November 11 was a club record seventh-successive defeat for the Robins.
Away wins against Middlesbrough and Sheffield Wednesday have followed, along with a 1-1 draw with Blackpool, prompting Foster to think the worst is over.
He told the Bristol Post: "There comes a point in every season where you have to stop and take stock, and that moment for us came after the Charlton game.
"It was arguably our worst performance of the season and, when you get to that stage, you know things cannot get any worse.
"We played terribly, lost to a team that were not that far ahead of us in the league and sat in the dressing room and knew we had reached our lowest point."
Foster continued: "We knew we had to regroup and results and performances since then suggest it was a real turning point.
"The performance against Blackpool helped and by the time we got to Middlesbrough, we were believing in ourselves again."