Villa have lost three of their past five games and have dropped to 14th place in the Championship following a five-game winless run.
"We were totally abject in everything we did apart from the first 20 minutes, where we had two or three chances and looked okay," Bruce told BBC WM.
"But as soon as they scored, everything fell to bits and that is where we are."
Former Hull City and Birmingham City boss Bruce, 56, was appointed in October with Villa 19th in the table.
He made an immediate impact and, after the Boxing Day win over Burton, had steered the club to 10th place - six points off the play-off places.
But Bruce admitted his side were now "having a tough time".
He added: "We cannot just wave a magic wand. We have to build and be strong in times like this.
"After an initial two three months where we showed sheets of recovery, the last month or so has been poor."
Bruce said the defeat against the Bees at Griffin Park was "arguably the worst away performance" since his appointment.
"There are times in your career where you feel embarrassed and that is one of them," he continued.
"We were so easy to play against. Every time Brentford passed it, they passed it through us. We weren't genuine in our work to get after them and didn't do enough.
"I can only apologise. I will try my utmost to make sure we start turning this around.
"We have to get a mentality and a toughness which gets you results in the Championship. We have to find a way - that's my job.
"Where we are at the moment is pretty sorry. We can only get better because we couldn't play any worse."
Aston Villa's display in Tuesday's 3-0 away defeat at Brentford was embarrassing, said manager Steve Bruce.