ATLANTA — Addison Reed has personified all that has gone right for the Mets’ bullpen. Acquired in a trade last season, he pitched his way into the team’s long-term plans, eventually taking on the role as setup man to closer Jeurys Familia.

He has become a trusted weapon for manager Terry Collins, who called upon him Thursday night with the Mets desperately hanging on to a one-run lead against the Braves.

But in the eighth inning, Reed paid dearly for a 91-mph fastball he left up in the zone. Adonis Garcia hammered it to leftfield over the futile leap of Michael Conforto for a two-run homer that sent the Mets to a 4-3 loss.

“Everything felt fine,” said Reed, who was working for the third night in a row. “I just made a stupid pitch.”

An inning earlier, Conforto appeared to save the Mets. Not known for his defense, he uncorked a powerful throw to cut down the tying run at home.

Travis d’Arnaud leaped and pointed to Conforto. That the outfielder still was in the game was a lucky break. Earlier, he banged his left knee against the side wall while making an inning-ending catch. He hobbled off the field with a trainer but stayed in the game.

The play appeared to provide enough momentum to give the Mets a victory over the cellar-dwelling Braves, who swept the Mets in a three-game series earlier this month. Instead, it was a footnote in a frustrating night.

“We’re having a tough time getting them out,” Collins said. “They’re playing very, very well.”

Freddie Freeman began the eighth with a single off lefty specialist Jerry Blevins. After Blevins struck out Nick Markakis, Collins summoned Reed, who began the night with a 2.48 ERA, had allowed only six earned runs in his previous 27 innings and had held righthanded hitters to a .200 average this year. Nevertheless, he surrendered the lead on one swing by the righthanded-hitting Garcia. It was the first time all night that the Mets had trailed.

“He wanted to go up above the strike zone to give him a different look,” Collins said of Reed’s 0-and-2 fastball that sailed high but not high enough.

Matt Harvey allowed eight hits in six innings, far from the dominance that he had flashed more often of late. He walked none but struck out only three. “It was definitely a struggle,” he said. But he held the Braves to two runs.

The previous time Braves righty Matt Wisler faced the Mets on May 3, he allowed one hit in eight shutout innings. But Thursday night, the Mets did not grant him an encore.

In the lineup for Yoenis Cespedes, who has a sprained wrist, Alejandro De Aza laced an RBI double in the second, the first of his two hits. He had begun play hitless in his last 20 at-bats.

Neil Walker had a pair of RBIs with a sacrifice fly in the third and a single in the fifth. But the Mets mustered nothing more, blowing a chance to pull within three games of front running Washington. The Nationals, losers of five straight, were idle.

In Harvey’s previous start, he allowed four runs and seven hits in six innings. It was the first time in three starts that he had allowed more than one run. But Thursday night, he kept the damage to a minimum. A.J. Pierzynski burnished his reputation as a Mets tormentor, finishing with two run-scoring hits. Otherwise, Harvey worked around his issues.

Harvey allowed consecutive singles to start the game but eased some pressure by picking off Jace Peterson. He stranded a pair of runners in the third. After Pierzynski’s RBI single with one out in the fourth, Harvey retired the next two batters to stifle a potential rally. After Pierzynski ripped a two-out RBI double in the sixth, Harvey got Erick Aybar to pop out.

Still, damage control wasn’t enough.

Said Harvey: “You’ve kind of got to flush this start and move forward.”