The Catalan giants had just the Supercopa de Espana to show for their efforts at the end of last season, after losing out in the three other competitions to Madrid's top two clubs.
Atletico pipped them to the Liga title with a 1-1 draw at Camp Nou on the final day of the season, as well as dumping them out of the UEFA Champions League at the quarter-final stage.
Fierce rivals Real, meanwhile, beat them 2-1 in the Copa del Rey final and coach Gerardo Martino was replaced by former player Luis Enrique after the season's conclusion.
"The main thing is to have a good attitude," said Bartra.
"I don't believe Tata [Martino] lost the team. The beginning was very good but we began to lose games and there were many off-field issues, although that shouldn't be an excuse. We have to be self-critical so there isn't a repeat.
"Tata trained as always and the team honestly didn't look so bad physically. On the contrary, we were at our peak and we didn't win because mentally we weren't as we had to be.
"We weren't consistent. We were able to do very good things, but then we weren't decisive in matches when we needed to be.
"We have to move on knowing these things. We appreciate it isn't easy to win titles and we have to be 100 per cent because if we're not, any team can move ahead."