EFL Cup winners City were on course for four major trophies this season until they were eliminated from the Champions League quarter-finals on away goals against Tottenham in April.
Their exit came in the most heartbreaking of ways after Raheem Sterling saw a stoppage-time strike ruled out following a VAR review in a dramatic second leg.
Guardiola's team have since held off the challenge of Liverpool to retain the Premier League title and can become the first English men's team to complete a domestic treble if they beat Watford in the FA Cup final at Wembley on Saturday.
The former Barcelona coach, who also got his hands on the Community Shield for the first time this season, is not convinced all four trophies could have been theirs, though.
Asked if he still thought the quadruple was impossible given how close his team went this season, Guardiola replied: "Yeah, the biggest one is far away.
"Always I thought after going out to Tottenham in the Champions League quarter-finals, if we'd go through would we have been able to beat Tottenham, go to Old Trafford, Burnley and win the points we needed to be champions against this Liverpool? Honestly, I don't know if we would have been able [to do it].
"Focusing on Ajax here, then going to Amsterdam to play in the beautiful city with Johan Cruyff's new generation there and maintain it [in the Premier League], with Tottenham at home, [Manchester] United, I don't know.
"It's incredibly difficult to win it [the quadruple] but we can win it [on Saturday] - the little quadruple. It's a big chance for us."
Guardiola twice led Barcelona to Champions League glory but was knocked out at the semi-final stage in three straight seasons at Bayern Munich.
The 48-year-old is yet to guide City beyond the quarter-finals and acknowledged he needs to work on his approach if he is to achieve further success in the competition, where he has seen Lionel Messi, Thomas Muller and Sergio Aguero miss crucial penalties during his coaching career.
"Of course, it's our dream to do it but I have to improve, I have to see what I can do better to do it. As a manager I'm not good enough in this club to maintain it," said Guardiola.
"In the same point it's a more difficult competition, except [Zinedine] Zidane who is a master in this competition.
"For all the other managers who stay 10-15 years, they don't win every season this competition. It's a tough competition.
"It's so tight. You go through or not go through. Leo [Messi] missed a penalty [in Barca's 2012 semi-final against Chelsea], Muller missed a penalty [in Bayern's 2016 semi-final against Atletico Madrid], Sergio missed a penalty [in this season's first leg against Spurs]; these kind of little details in the domestic leagues you get another chance.
"We cannot imagine what we would have done without [Aymeric] Laporte in this competition. He is the best signing we have done in the last four or five years - a young player, personality, he has everything. But against Tottenham, two balls and we conceded two goals and in that competition you are out.
"That's why it's so difficult, we have to accept it. We will work on I don't know what because we tried to do our best, but the Champions League is difficult and I can admit it. It's not the same level as we've achieved in the other competitions."