With Arsenal sitting in sixth place and six points back of the top four, Bellerin bemoaned a lack of initiative as the Gunners controlled possession in the first half.
"We waited too long," Bellerin told Arsenal's official website. "In the first half, we were waiting for something to happen. We didn't create chances, we didn't have the ball like we should have done and then in the second half we paid for it.
"We need to take the initiative a bit more, to attack more and keep the ball because we didn't do that today.
"In the second half, at the end they were tired, they weren't pressing as much and we were creating a bit more space so we had chances. It's a bit frustrating because you could see that we could have gone home with a point or even three.
"I think we weren't at the standard required, so it's something to look at for the next game."
Arsenal return to the field for a Europa League last 32 first leg against Ostersund on Thursday, with the second leg set for the Emirates Stadium one week later.
"There's still plenty of games to play," Bellerin said. "There are loads of points still to play for, so we need to look forward. We can't just be stuck looking back at this game."