Cook's late equaliser, a fine curler from 20 yards, secured a 2-2 draw for Russell Slade's men after goals from Paul Hayes and Clayton Donaldson had cancelled out Martin Rowlands' opener.
But with the Bees down to 10 men after Jake Bidwell was sent off and David Mooney having a penalty saved by Simon Moore, Cook knows they missed a chance to grab victory.
"We should have won," he said.
"We were disappointed at half-time, we thought we had outplayed them and out-passed them and they had two corners and scored both.
"The gaffer went mad at half-time, saying we need to defend them better because all our good play was for nothing so it was important we put that right in the second half.
"I know they'd had a man sent off but I think we would have got something out of the game anyway."