Having overcome a 4-0 first-leg deficit against Paris St Germain in the previous round with a 6-1 win at home, Barca contrived to lose 3-0 at Juventus in their quarter-final first leg in Turin.
"This time, it's more difficult to believe in a comeback," said Luis Enrique. "It was like reliving a nightmare... I can't get the first half out of my head."
Paulo Dybala scored twice to put Juve 2-0 ahead inside 22 minutes and defender Giorgio Chiellini headed in the third early in the second half in a rousing performance by the Italians.
"It was a very difficult first half, very similar to the one in Paris, and although we improved after halftime, the first half was full of problems."
"It's difficult for a coach to try and do everything right for the team and then realise that he hasn't succeeded," he added.
"I don't really want to think about it now. Tomorrow will be the time to get up again and think that we have to score three goals to level the tie and four to win it."
"We have to put some brakes on our enthusiasm," he said. "Of course, the lads have produced a great performance, both in attack and in defence.
"But it's just the first step. In Barcelona, it will be different and we have to try and score a goal."