A late free-kick from substitute Vicente Rodriguez was enough to give the hosts the points and condemn Hull to just their third defeat in 15 games.
"I'm frustrated. The first half was a non-entity in that we cancelled each other out but it was a very good game in the second half," said Bruce.
"We created great chances and could have gone 2-0 or 3-0 ahead. But we didn't take them and that is our Achilles heel.
"Bearing in mind we have two strikers out (Sone Aluko and Matty Fryatt) we've done remarkably well and in the second half we were the better side by far.
"A lot of teams are evenly matched in the Championship and we can finish in the top six. I hope that doesn't alter.
"It's cruel to be without our two strikers, but we are doing admirably well without them."