The midfielder was critical of his side's approach after Inter suffered a 3-1 defeat to AC Milan in Saturday's derby, their first league loss of the Serie A season.
Barella acknowledged that the problem has been apparent for some time, with Inter Milan too often relying on their ability to improve as matches progress.
"Certainly, if we are talking about this, it is something we have been missing since the beginning of the year. At this moment, it is an obvious problem for us," he said.
"It is true that on many occasions we have shown our strength, perhaps coming through in the second half or when we manage to get going, but clearly not every match follows the same pattern.
"You cannot expect to repeat the same script every time, especially when we have a packed schedule with matches every three days. The physical and mental fatigue is beginning to weigh heavily, and we have to learn to manage games better without being forced to take them all the way to the 90th minute with the result in the balance."
Inter's defeat at San Siro also continued a difficult period in the fixture against their city rivals. Barella has experienced more successful derby runs during his time at the club, but admitted the team's performances have become unpredictable.
"I have experienced it from the other side as well. I had a run of many consecutive derby victories, so I do not know whether it is something cyclical, but certainly in recent years we have not approached these matches in the best way.
"There have been derbies in which we played well and lost, and others in which we played badly and perhaps drew. We are too inconsistent in derbies and, at the moment, also in important matches which we have managed to win but where we should still have performed better."
Barella rejected the suggestion that Inter simply needed to prove they were the strongest team. He argued that reputation and perceived quality would count for little unless the players corrected the weaknesses appearing in their performances.
"Being the strongest does not always count, because it has happened before that we were perhaps the strongest team, or the team everyone considered the strongest, but in the end we did not win.
"At this moment, more than concentrating on our strength, we have to focus on something else."
The midfielder pointed to individual mistakes and a decline in collective defensive solidity as areas requiring attention. Inter's ability to defend as a unit had been central to their previous successes, including reaching the Champions League final and winning the Serie A title.
"Many goals in recent matches have come from individual errors, and we have to improve those ourselves, each one of us, because unfortunately individual mistakes hurt the team.
"But as a team we have to improve our solidity, which has always distinguished us and helped us reach the Champions League final and win the Scudetto. Perhaps at this moment we are not the best version of ourselves.
"We still have the desire to win, absolutely, because that is what gets us out onto the training pitch every day and gives us motivation, but we have to improve."