The Tykes had gone ahead through Aaron Leya Iseka but two goals from Andreas Weimann late in the first half turned the game on its head and proved to be enough for the Robins to take all three points.
It was the hosts' first win at home in 18 attempts and Andersen feels the Tykes need to be more clinical at both ends of the pitch to halt their poor run of form that has saw them lose seven consecutive league games.
"Of course, being back was great, I’m happy with that, but I’m still feeling disappointed because I feel that we deserved more from the game today," he said.
“But we need to do more. Maybe we can be more clinical in both boxes. It wasn’t great to give two goals away like we did in the first half.”
The result leaves the Reds second bottom, three points ahead of Derby County who have been hit with a 12-point deduction for entering adminstration.
The sides face off on Wednesday but Andersen says there is belief within the squad that they can climb away from the relegation zone.
“I’m still confident," he added.
“If we all stick together, support each other and take the positives out of today’s performance, particularly in terms of the chances we created today, we can feel confident that it will turn for us.
"We have to remain positive, keep cheering each other up, and continue to work hard every day and we will make our own luck.
“We all have each other’s back here, and we’ll regroup for Wednesday night now. I’m happy to be back out there, my legs feel good, I feel strong.”