Saints went ahead through Danny Ings but copped two second-half goals, including a last minute winner from Son.
"I mean, the way we lost the game we cannot be more hurting to be honest," said Hasenhuttl.
"In the first half we must be three up and in the second half, especially the last 30 minutes, we had not the game we wanted to have.
"The way we give them the goals, it's always the same issue.
"We don't clear the ball after the corner, we make a stupid foul around the box in the last minute and then you concede a goal that you normally never can concede.
"Then all the work you put in is then gone and this is hard to take."