Sam Vokes stepped off the bench to fire an added-time equaliser which denied Watford the chance to go second in the Championship.
An error from Matthew Briggs presented Vokes with the chance to snatch a point, and Zola believes mistakes at the back could cost him team automatic promotion.
"I thought we'd done enough," said Zola. "We came back from behind twice and then went ahead, and towards the end I didn't see that coming, but it's not the first time that has happened.
"A sloppy piece of defending costs us a lot. I understand we have a very young team, but there's not much time for them to learn, we need to grow up quickly.
"We have done very well so far and it would be a shame not to finish what we have done.
"It was a pity because Briggs played a good game. Today it was him but other times it has been someone else.
"Now is the time we cannot give anything away, every single game is going to be very difficult."