With three minutes to go the Biancoverde looked to have wrapped up a famous win against the Irish province when replacement scrum-half Fabio Semenzato dotted down and Kris Burton converted his try.
But Sexton struck a perfect drop goal in reply and Smith admitted there was nothing his side could do about the manner in which they lost the game.
"We played very well and I want to talk more about the positives of this game because there aren't many negatives," said Smith.
"The attitude of the squad for a start was good and our work this week has been great.
"We had courage in defence and only conceding one try against a team of this quality is good.
"But in every game there are regrets and I'm just sorry about the try.
"We played with the ball in hand very well, creating a lot of phases - the first action of the game almost lasted three minutes.
"The work we did this week we saw on the pitch and this is the path we want to go down. We'll put this behind us and go to the next game."
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