"We probably underestimated how much the emotional high of Trent Bridge affected us," Cook said after the big loss in London.
"How hard it would be to get back up to that level you really need to be at to beat Australia.
"It's a little bit disappointing but I'm not going to worry about it too much."
Cook highlighted the fact he and coach Trevor Bayliss always spoke of needing to win three games during the pre-series training camp in Spain.
"You can kind of understand it (playing so poorly in the last Test)," he said.
The two sides face off next Monday in a one-off Twenty20 clash at Cardiff.
A five-match ODI series follows, however a handful of England's brightest stars could be rested due to upcoming tours of the United Arab Emirates and South Africa.
Joe Root, Ben Stokes, Jimmy Anderson and Stuart Broad are set to skip the limited-overs action according to reports in England.
Ian Bell, who has played 115 Tests and debuted in 2004 like Michael Clarke, suggested he was unsure about his future in the sport in a post-match interview.
Cook hoped Bell would play on.
"What you say in an interview now might not be what you really believe deep down," Cook said.
"It's such an emotional time.
"I haven't actually heard exactly what Belly has said but he's a class player and I hope he's around for a few more years."
-AAP
