The Reds lost their round of 16 tie on penalties after losing at home 1-0 last night.
Robertson said: "That's the competition. Obviously we got the win last week without playing our best. I think we played so much better today. That's football, you end up being on the wrong end of the result. But you can't question the effort, the endeavour, everything that we put into the game.
"It was just two really good teams going at it and when it goes to penalties, it's always a lottery and unfortunately for us we're on the wrong side of it. I can't be prouder of the players, how much effort we put in, and it's just unfortunate that the efforts weren't rewarded."
"We've got a lot to look back on but I think there'll be a lot of positives from this game. Obviously it's a massive disappointment not going through."
The Scot added: "I think for finishing top of the group, we got the toughest tie of the whole Champions League. PSG are a fantastic team, got threats all over the pitch and if they play to that level then they should go far in the competition."