Championship Wrap: Forest beaten again
Forest's winless run stretched to 10 matches in all competitions after their poor home defeat at The City Ground - a result that saw them slip to 11th in the Championship table.
Pearce's men topped the league in September but - despite spending significantly in the close-season - have dramatically dropped off the pace.
Arsenal loanee Jon Toral gave Brentford a 17th-minute lead, punishing Forest's failure to clear.
And Andre Gray doubled Brentford's lead 10 minutes before the break when he made the most of a weak backpass from Forest substitute Eric Lichaj.
Brentford goalkeeper David Button made good saves to deny Danny Fox and Michael Mancienne in the first half, before the result was sealed three minutes after the interval.
Kelvin Wilson brought down Gray in the penalty area and Alex Pritchard made no mistake from the penalty spot.
That meant Michail Antonio's ferocious 82nd-minute effort was no more than a consolation and afterwards Pearce bemoaned his side's defensive mistakes.
"I think we are committing too many stupid individual errors at the back," Pearce told BBC Five Live.
"Until we eradicate these silly errors and give ourselves a chance to have a foothold in the game, we are going to struggle a bit.
"If you concede the first goal in any game, especially at this moment in time…that confidence…decent players are looking very average players.
"We have to turn things around."
Wednesday's other Championship match saw Fulham move out of the relegation zone after coming from two goals down to draw 2-2 with 10-man Blackpool.
Ishmael Miller and Jacob Murphy scored in the first 30 minutes for rock-bottom Blackpool, but Tony McMahon's red card just before the break hurt the visitors.
Scott Parker pulled a goal back on the stroke of half-time and Bryan Ruiz levelled with 17 minutes remaining, but Kit Symons' men were unable to find a winner.