Posh had withstood all that the promotion favourites could throw at them for over an hour of the game at The Hawthorns but succumbed to conceding two goals in the last half-hour of the Boxing Day clash.
"We just caved in which is the disappointing thing for me," the Posh boss said after the defeat.
"We were pretty comfortable for an hour. But it is becoming a bit of a trait now that when we come under a bit of pressure we concede, not just one, but two very quickly.
"It is something that needs addressing. We gifted West Brom two goals.
"For the first goal we let Graham Dorrans skip through three challenges which isn't good enough. You have to get tight to people and deal with it better than that.
"We have to stand up, roll our sleeves up and see it through until the pressure comes to an end."