It was the 11th time this season that the Daniels had let a winning position in an Evo-Stik League Division One East match slip from their grasp.
Joe Burgess' 12th goal of the season had given Stamford a first half advantage after a solid start to the clash at the Zeeco Stadium.
However, a super save from Nailers custodian Sebastian Malkowski denied the Daniels doubling their lead from Ollie Brown-Hill's 65th minute penalty.
Alex Peterson then headed Belper level with 11 minutes remaining before substitute Nathan Curtis cracked home a superb winning goal late on.
And that left boss Drury questioning some of his players as they dropped down to 11th place in the league standings after a frustrating campaign.
He said: "We had chances to clear the ball for both goals and both times we haven't headed the ball away.
"You can't defend like that. The defenders need to head the ball and it's job done. The defending was poor.
"But that's also two penalties we've missed in the last two home games and you afford that to happen. It has killed us.
"The game would have been done at 2-0. They would have been really disheartened, but we've given them a chance to stay in the game.
"Then a ball into the box has come off one of our players and gone into the far corner which has given them the belief and put us on the back-foot.
"People moaned last season about Henry Eze and Del Gordon sitting deep, but they wanted to win the ball.
"If it went into the box, they wanted to win it and I'm afraid some of them just haven't got the heart to do that.
"I think that's been the story of the season. You have put your body on the line. It just proves how much Henry and Del did for us last season.
"It's really made me think about the rebuilding for next season. I said to the players afterwards that the numbers I had are slowly dwindling because they're just not doing their jobs.
"It's a total rebuild for next season. There's too many players who have a seven-out-of-ten one week and then a three-out-of-ten the neext which is not what I want."
Stamford began with the same starting line-up that had been so impressive in their last home outing against Tadcaster.
They were also boosted by the return of Harry Peasgood and Joe Boachie to the squad against a Belper side who had seen their play-off push falter in recent weeks.
Gregg Smith struck an early shot wide for the home side after a smart turn and shot before Danny Gordon dragged an effort wide at the other end from George Milner's long throw.
Piteu Crouz then went closest to breaking the deadlock when he weaved his way into the Stamford box only to see his effort hit the post after beating Dan Haystead in the home goal.
It was then Stamford's turn to apply the pressure with some neat attacking interplay and they took the lead three minutes before the break.
Liam Adams picked out Smith whose header was fumbled by Malkowski and Burgess pounced to fire home his 12th goal of the season.
Stamford kept the advantage until the break and then created the better openings early in the second period.
Rob Morgan saw a decent strike crucially blocked by Nailers skipper Danny South before the marauding Daniels midfielder made a sublime surge into the Belper box where an intricate piece of skill saw him upended by Isaac Assenso.
However, Malkowski made amends for an earlier error with a fine save from Brown-Hill's spot kick as he plunged low to his right to turn the effort away.
Stamford still created the better openings with substitute Boachie inches away from connecting with an inviting Jake Duffy cross before the same combination joined forces with Boachie heading narrowly over from another Duffy delivery.
Crouz then dragged a decent opening wide as Belper began to sense they could still get something from the game.
And their leveller arrived with 11 minutes remaining as Gordon's free-kick into the box wasn't dealt with and Peterson headed home the equaliser.
The goal rocked Stamford and Peterson could have put Belper into the lead minutes later but he headed over when well-placed.
But the inevitable winner for the visitors did come with just four minutes remaining as substitute Curtis pounced on an indecisive clearance before delivering a superb shot which flew over Haystead and into the home net.
There was no time for Stamford to respond as the Daniels slipped to their fifth home league loss of the season having lost just once at the Zeeco Stadium last term.