Glasgow Warriors coach Gregor Townsend said he thought they had done enough to close out victory against Northampton.
Harry Mallinder scored a late try to snatch a 19-15 win against Warriors in Sunday's European Champions Cup match.
"To lose in the last minute of the game is disappointing, but it's really disappointing for the players," Townsend told BBC Sport.
"They're the ones that put the effort in and there was huge effort right throughout that game."
Glasgow, who suffered a fourth straight defeat across domestic and European rugby, had been reduced to 14 men when replacement lock Tim Swinson threw a punch with five minutes left to earn a yellow card.
"Even when we gave them possession I felt we were defending really well," continued Townsend.
"The spirit and character was on show, but it was an excellent kick in the end that proved the difference and well done to Northampton for keeping in the game."
He added: "We haven't got the fluency, probably for a number of reasons.
"We had players away at the World Cup, we had a couple of games postponed. Obviously the playing group are not doing something as well.
"Today was much better and we need to build on that performance and work out how to win as well, which we came very close to doing today."
|Glasgow's losing run:|
|17 Jan: Northampton 19-15 Glasgow (ECC)|
|9 Jan: Racing 92 34-10 Glasgow (ECC)|
|2 Jan:Glasgow 11-14 Edinburgh (Pro 12)|
|27 Dec: Edinburgh 23-11 Glasgow (Pro 12)|
Flanker Chris Fusaro says he is "heartbroken" that Glasgow are lall but mathematically out of the running for a quarter-final spot in the European Champions Cup, but that they made it hard for themselves by not playing to their best throughout the campaign.
With head coach Townsend conceding his side will not get one of the three best runners-up spots available, despite it being potentially possible if six results go their way, Fusaro was left to rue another season of European disappointment.
"I truly believed that we were going to win so to lose it so late is heartbreaking," he said. "It's another year of not making it to the quarter-finals and that was a massive goal of ours."
As well as a disappointing European campaign, the defending Pro 12 champions lie in the bottom half of the Pro12, and Fusaro believes other teams are raising their game to play the current league champions
He said: "Gregor (Townsend) said at the start of the season that becoming champions is hard enough but maintaining it is even harder. We've found that but we've made it hard for ourselves.
"Doing well in Europe was a target but it is another level. It goes up from playing in the league. You have to be at 100% of your potential to progress. It's massively frustrating."