Francois Louw believes his mistakes cost Bath in their 18-13 Heineken Cup defeat by Leinster on Sunday.
The South Africa flanker initiated the move which produced Matt Banahan's try and gave them a one-point advantage.
But Louw, 26, went on to concede a penalty, which was converted by Jonny Sexton to restore the visitors' lead, and was then shown a yellow card for infringement in the dying minutes.
"I let the boys down by giving away a penalty," he told BBC Radio Bristol.
"It gave Leinster the chance to put the kick over which put them in the lead. In games like this it's unacceptable.
"It's those little, cynical things we've got to rub out of our game in order to get back onto the winning track."
The defeat hindered Bath's chances of progressing in the competition as they have now lost two of their three games so far and sit third in Pool Three.
The return leg against defending champions Leinster is only six days away, but Louw is confident they can get a result.
"We need to get back into this tournament with a win under our belts," he added.
"It's going to be massive but that's what the Heineken Cup is all about. The belief is there so the boys are going to pull together this week and we're looking forward to getting out there."