The Ligue 1 powerhouse won the tie on away goals after the two legs plus extra-time finished 3-3 at Stamford Bridge on Wednesday, despite playing almost 90 minutes with one less player.
The Portuguese tactician, who saw his side twice lead on the night, said Laurent Blanc's men deserved to progress to the quarter-finals.
"Clearly [PSG were worthy]. Opponent was stronger than us, coped better. Our performance was not good enough," Mourinho said.
Chelsea played against 10 men for the best part of an hour in normal time, and the entirety of the additional 30 minutes, after PSG striker Zlatan Ibrahimovic was sent off just after the half-hour mark.
Mourinho said his players' mental strength was lacking to put the undermanned visitors to the sword.
"That's my feeling [Chelsea players not mentally strong enough]. Collectively they were better than us and I don't have much to say," he said.
Not wanting to lament further on his side's lost chance to win a trophy treble, Mourinho said they must refocus on the Premier League - the only title Chelsea can win for the remainder of the season, having already secured the League Cup title.
"We need to react. We have the Premier League to win. That's what I've told the players. It's not a time to cry," he said.
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