Saints lost 4-0 at Fir Park on November 30 to remain bottom of the Ladbrokes Premiership.
Motherwell profited from a number of crosses into the box and Saints manager Tommy Wright admitted his team capitulated after losing the opener.
But Saints go into Wednesday's visit of the Steelmen in eighth place in the table and only three points off the top six after losing just twice and keeping six clean sheets in their subsequent 12 matches.
Kerr feels he and his fellow defenders have heeded the lessons of their Fir Park experience and can push for the top six.
The 23-year-old, who was reunited with fit-again former youth-team colleague Liam Gordon soon after the Fir Park game, said: "That was probably the lowest point of the season.
"You saw the goals we conceded against them, I think it was four crosses or set-pieces. It was really embarrassing for us as defenders to concede such poor goals.
"We knew that something had to change and it was simply defending. We have done that and have a few clean sheets on the board since then and points in the table.
"The gaffer showed us what we could do better at every goal and it was simple things. I feel we have put that right and are doing the simple things better."
Saints' early-season struggles were particularly difficult for Kerr to handle.
"It was really tough for me in my first season as captain," the centre-back said. "We were losing a lot of games and conceding a lot of poor goals as well.
"It was hard at the start of the season but we have come into a good run of form.
"I don't think us or the gaffer were ever really worried at any stage of the season because we knew if we got a few wins going and some momentum we could go on a good run.
"It's good to get some confidence in the squad. I feel we are scoring a lot more goals now as well so the strikers are full of confidence. It's a good match.
"Hopefully we can continue our league form and get into the top six because it's really tight there."