Mohsni had a gilt-edged chance to score towards the end of the game against Raith Rovers at Easter Road earlier this month.
But he headed over from just a few yards out, then looked on in agony as the Kirkcaldy club snatched a late winner in extra-time.
The French-Tunisian defender is determined to atone for his error in his side's final games - against Stranraer and Dunfermline.
He said: "I was very disappointed in the cup final because I had three chances.
"The best chance was in the last minute and it was hard to put my body in the right position to score. I was very disappointed.
"All the players jumped in front of me and the ball came onto my head. I didn't see it.
"The defender jumped in front of me and I headed it late.
"I don't need to see it back, I know it was bad. I had to score but it was concentration.
"I don't know if the goal post has recovered - maybe in two seasons, when we play Hibs, I will see.
"I think that we played better than Raith Rovers but they had one chance and scored. It was very frustrating."
He added: "But going undefeated is our last achievement now. It would be fantastic.
"The manager said in the changing room it would go down in the history of the club.
"It is just Arsenal that have done it in recent history, so it would be very good."
Mohsni took his tally for the season to 12 with two goals in the 2-1 win over Ayr United at Ibrox on Tuesday night.
And he is looking to beat his best-ever haul for a campaign by netting on the next two weekends. He said: "I scored 13 with Southend United. I will try to score more."