Alastair Cook's team have found themselves playing 10 back-to-back Tests against Australia in the space of six months - and as they near the end of their odyssey, with the urn lost already Down Under, Pietersen concedes they have perhaps become "mentally a bit fragile".
England lost the Ashes before Christmas, after three successive and wide-margin defeats, but have fared better to date in the Boxing Day Test in Melbourne.
Before start of play on day three at the MCG, Pietersen told Sky Sports: "Scoring's been incredibly tough. It's been a series of hard work.
"I think people mustn't forget as well - it's never been done before - that guys have got to go and endure 10 back-to-back Ashes Test matches.
"I've been told a number of post-Olympic athletes go into some sort of depression or negative frame of mind.
"I'm not saying we are in some sort of depression or negative frame of mind - but after you've competed at such a level, post that competition, mentally you are a bit fragile."
England arrived in Australia more than two months ago, hoping to win the Ashes for a fourth successive time - but in the end, Pietersen senses, it was asking too much.
"To play an Ashes and then another Ashes, and for us being away from home, it's a tough gig.
"We just want to salvage some pride - win here, we hope, and try to do something special in Sydney.
"It's been an incredibly pressurised situation. Playing back-to-back Ashes series and being away for the second leg and not starting in the manner we are accustomed to... has been incredibly difficult."
Pietersen top-scored in England's first innings in Melbourne, only to fall to a bizarre shot when he was bowled trying to counter-attack Mitchell Johnson.
"Yesterday's dismissal, I know there's a bit of a talking point," he said.
"As soon as (Tim) Bresnan got out, I was under the impression I had to do all the scoring - take all the strike.
"Look, I get out for nought, I get nailed. I get out for 70, I get (nailed).
"I was playing for the team's cause and trying to score as many runs as possible as quickly as possible - because I knew I was going to have to do it."