Bruce's Albion are still waiting for their first league win of the campaign - as they were held to a frustrating goalless draw.
Albion were the better side in the first half with Jake Livermore and Jed Wallace testing keeper Ryan Allsop.
Conor Townsend and Karlan Grant both had sights of goal - but until a late Semi Ajayi header they failed to really test the Cardiff back line.
Bruce said parts of his side's display were a throwback to last season - but they need to draw a line under it and go again Saturday.
He said: "We didn't have any real quality, we made the wrong decisions.
"I couldn't fault the effort and endeavour, the quality of the decision making was the disappointing thing.
"We were bright in the first half, we had a good 15, 20 minutes, but we became sloppy and gave it away.
"We didn't create enough and that was our big problem.
"It was chalk and cheese from Watford, and that is the level we need to stay at.
"We have to hold our hands up and say we did not play well enough to win the match.
"At times it felt a bit like we were for large periods last year - but we have to draw a line through it and get ready for Saturday."
Albion's need for a centre forward was again evident - with Karlan Grant having a poor evening and Bruce lacking attacking options on the bench.
Earlier in the week the Baggies were in for Man City striker Liam Delap - who has since joined Stoke on a season long loan, where his father is currently first team coach.
And Bruce revealed that is what swayed the decision.
He added: "It was the influence of his Dad.
"We tried, we did our best, there was nothing more we could've done and the boy chose to go to Stoke.
"It was the influence of his father, unfortunately for us."