The Gulls went down 1-0, having scored nine goals in their two previous games against high-flying Forest Green.
"They went away from the game plan and started going off script and they beat themselves," Nicholson said.
"I'm very disappointed with the application of the lads throughout the week and they owe the supporters and me a hell of a lot more."
The loss kept the Gulls in 17th place in the National League, just five points above the relegation places.
"They turned up to training on Wednesday looking like they've made it after two good performances and after a good run of form," Nicholson told BBC Radio Devon.
"But we are not good enough to turn up on a Saturday at 80%, we have to be at it. If we're at it we can beat anybody.
"But if we turn up thinking we've cracked it, that's what you get."
Torquay United manager Kevin Nicholson says his side's poor build-up was the reason they lost to Boreham Wood.