Speaking with former Australia goalkeeper Mark Schwarzer, Posetcoglou also conceded Spurs fans are losing faith in his project.
"'It's been a tough season. There's no hiding from it. It hasn't gone anywhere near the levels we wanted," he told Optus Sport.
"There's frustration in that we've seen glimpses of what we want, but then enormous setbacks.
"But in the league, we have been nowhere near it, it has been disappointing."
On the biggest challenge this season, Postecoglou continued: "I think we got the start of the year wrong.
"It's become increasingly challenging for footballers, they don't get the traditional break.
"I think we went into the season really hard and underestimated the challenges of Europe with the two extra games, having a cup run... all those things."
However, Postecoglou can still find positives from a difficult campaign.
He added, "Surviving tough times can often unite people.
"Because you think there's nothing down the track which will be anywhere near as bad as what we've gone through. We've survived and we've stuck together.
"I guess the biggest way we can influence our fans is by giving them hope, I know some have lost faith in what we are trying to do.
"We have still got things to play for in Europe and we've got to try and finish the season off in a positive way."