Birmingham went into the derby clash in a good run of form in the Championship, but the disappointment at Villa Park has been followed by setbacks against both Millwall and Brentford.
Cotterill watched his side succumb 5-0 at Griffin Park on Tuesday evening, and the boss of the West Midlands outfit has acknowledged that witnessing his team lose heavily had been "very painful".
The 53-year-old told BBC WM: "A very painful night tonight. You can't make those mistakes we've made and expect to get anything out of a Championship game - especially away at Brentford.
"Some of the mistakes we've made tonight are uncharacteristic of those boys and it put us on the back foot right from the off. We just can't give away the goals we've given away tonight let alone the missed chances.
"We've had some tough results this season, we turn a corner with some positives ones, but since the defeat at Villa it's hard picking the players up again."
Birmingham sit in 20th place in the league standings, just one point above the relegation zone.