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Leading 5-3 from the first leg of their last-16 tie, City fielded an attacking XI in the second leg on Wednesday but lost 3-1 to go out on away goals.
"I tried to convince them in all the meetings we had to come here, try to attack and score," said Guardiola.
"My mistake was being not able to convince them to do that."
The Spaniard added: "I did [convince them] in the second half but it was too late.
"All managers make mistakes but I don't think it was down to a tactical mistake.
"It's simple. The difference was between the first and the second half. In the second half we tried to win the game, we tried to play. I did it all my career in that way. But the problem was the first half. We weren't there."
City were overrun and sloppy at the back as they conceded twice in the first half.
They were much improved in the second 45 minutes and looked to have saved themselves through Leroy Sane's 71st-minute strike, but further defensive frailties were exposed as Tiemoue Bakayoko headed in a decisive third for the home side.
Guardiola continued: "It's not about the defence. Today was not about that. Why was the second half a problem with the defence?
"Our strikers have to be aggressive and pick the ball up, but we didn't at this crucial time. That's why we are out."
This is the first time that a side managed by Guardiola have exited the Champions League at the last-16 stage.
But he was adamant his players would learn from the experience and come back stronger next season.
"I came here to win the Champions League. I tried, I tried - and I will try again," he said. "Playing like we have done this season, like in the second half, would have been enough.
"The competition is so demanding. Hopefully we are going to learn so that, next season, we can come back here and make the same performance we did at the Etihad for the whole 90 minutes."
Former Manchester City winger Trevor Sinclair on BBC Radio 5 live: "I thought Kevin de Bruyne, David Silva and Fernandinho didn't take responsibility on the ball. They looked shell-shocked at the start by Monaco. From the first kick of the second half they went long and the gaps appeared. They needed to realise that sooner. In the first leg, Yaya Toure did that. It happened too late here.
"There was a lack of leadership on the pitch and a lack of bravery. Take John Stones out of that, who was excellent tonight. There was too many players who underachieved and didn't take responsibility."
"Looking at the game management, Pep may feel let down by his players. They didn't have the footballing IQ to know they had to play some long balls in, to recognise the scenario of the game. It took Pep to tell them at half-time. That is basic football."
Former Manchester United defender Phil Neville on BBC Radio 5 live: "They have been two brilliant football matches. City lost it in the first half when they were outplayed, outfought and were bullied. They got back into the game and they thought they were through. But their Achilles heel was a sloppy goal. Another year has failed to live up to expectations.
"I am not so sure the signings over the last five years have been that good. But when Sane scores I think Pep is thinking 'this is perfect, we can control the game from here'. Then they concede.
"Pep Guardiola doesn't go out there and spend the bulk of his money on defenders. He will be thinking if they had put one of those chances away they would be in the next round."
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Pep Guardiola says his failure to convince his Manchester City players to attack in Monaco is the reason for their Champions League elimination.