Sam Vesty has told Saints they must learn quickly after their errors allowed Wasps to win 36-32 at Franklin's Gardens on Saturday afternoon.
The lead changed hands nine times as both sides continually gave each other routes back into the game.
Taqele Naiyaravoro was yellow carded twice, making a red, with the second incident - a high tackle - giving Wasps a penalty try that won them the game.
Saints scored five times in all, with James Fish, Naiyaravoro, Mitch Eadie and Fraser Dingwall adding to a penalty try.
But missed conversions from James Grayson were costly and so were the errors that crept into the game, including Naiyaravoro's actions.
And attack coach Vesty, who was given the chance to lead the team as part of Chris Boyd's Premiership Rugby Cup coaching rotation system, said: "We're bitterly disappointed.
"We showed some signs of playing really quite well and scored some good tries but we let them off the hook.
"We gave cheap possession away and we gave cheap territory away at completely the wrong time.
"You can't afford to do that and they are a dangerous team who can score.
"We kept letting them get back into the game all the time.
"We've got to manage those times better and we've got to manage our way out of areas better.
"If you keep giving them the ball back, it's going to hurt you eventually - you will concede tries.
"The frustrating thing is that some of it's our process stuff. We can all drop a ball but when we don't do what we say we're going to do, it's really frustrating.
"There are some positives in that we moved the ball well two or three times and you saw what we could do, but the amount of times we gave the ball back to them we didn't give ourselves enough momentum to keep doing it.
"We've got to learn, and we've got to learn pretty quick."
There was no doubt that Naiyaravoro's sin-binnings were costly as Wasps scored twice while he was off for what was viewed as a deliberate trip during the first half, and his second yellow cost them that decisive penalty try.
But Vesty was in no mood to complain about the decision making of referee Michael Hudson.
"Sometimes you get those decisions, sometimes you don't," Vesty said.
"I never criticise the ref because they do a cracking job and make lots of decisions.
"We always harp on about the one that goes against us."