Successive 2-1 defeats to Italy and Uruguay mean that the Three Lions will exit the tournament after just two of their Group D encounters.
It is the first time that England have failed to progress from the group stages of a World Cup for 58 years, and the 28-year-old says that it is "very tough" to accept.
"A few weeks ago it was one of the highlights of my career and a couple of weeks down the line it's one of the lowest points," Cahill told reporters.
"It's the worst feeling I've had as a player. There's nothing more I can say. It's very tough to take, football can be cruel sometimes."
England finish off their campaign by facing already-qualified Costa Rica on Tuesday.