Prosperity can be fleeting. For the Mets, it’s often gone in an instant — especially when their flammable bullpen is involved.

In the span of 24 hours, they went from feeling good, headed to what looked like their sixth victory in eight games, to getting swept in a makeshift doubleheader and once again fading away from .500 and falling a season-high 7¹/₂ games behind the red-hot Braves in the NL East.

“In the moment, obviously, it brings you down,” manager Mickey Callaway said. “I’m sure everybody’s feeling pretty crappy right now, but they’ll come in tomorrow like they always do and battle and grind.”

In both instances, the bullpen delivered the dagger, ruining potential victories. First it was closer Edwin Diaz, who coughed up the lead Thursday night before the game was suspended due to heavy rain, and then, after the game was resumed Friday, allowed a run-scoring single to Paul DeJong in the 10th inning of a 5-4 gut-punch defeat.

DeJong got to another of the Mets’ arsonists in the nightcap, homering off Jeurys Familia to pull the Cardinals even in the eighth, and Dexter Fowler followed soon after with a three-run shot off the former closer to cap the ugly inning and send St. Louis to a 9-5 win.

Because Diaz pitched in the opener, he was unavailable for the second game. In addition, Robert Gsellman had a tight back after pitching a scoreless seventh and Callaway was holding Seth Lugo back for a save opportunity. So it fell onto Familia’s plate to pitch the eighth.

“He’s still the guy — he’s the guy that has to have success there,” Callaway said of Familia, who was signed to a three-year, $30 million contract in the offseason. “He’s been pitching good. Just because they tie the game, doesn’t mean you yank him out and put someone else in.”

Just two pitches into the eighth, after Wilson Ramos had given the Mets (33-36) the lead, it was gone. Familia was taken deep on a 1-0 slider by DeJong. Even after Familia allowed a two-out double to Yadier Molina and walked Harrison Bader, Callaway stuck with the right-hander, which backfired. Fowler took him deep, resulting in a stadium full of boos as Familia’s ERA rose to 6.91.

“The confidence hasn’t been the issue for me,” Familia insisted. “I have been frustrated at times when a teammate of mine does his job and then when they turn it over to the bullpen, I blow the game or the bullpen blows the game. That’s what has me frustrated.”

It ruined a rally from 4-1 down. The Mets’ relievers have proven good at that, wasting strong starting pitching performances and clutch rallies. Friday’s ugliness was just the latest example. The team’s relievers have now blown 16 saves, the most in baseball, and have an 8.84 ERA in their past 14 games while being saddled with five losses. When asked if they have enough quality depth, Callaway said, “I think we’re fine there.”

On a recent 2-5 road trip, the bullpen gave up two big leads in crushing losses. Diaz blew a three-run lead against the Dodgers on May 29 and the bullpen also flushed a 5-1 lead against the Diamondbacks on June 1.

“You want to win those games. You got to win them if you’re a good team, but we didn’t get the job done,” Callaway said after the opener. “We’ve had a few of those in the last couple of weeks that have hurt us.”

Minutes after the Game 1 defeat, Pete Alonso responded to a tweet by Kevin Clancy of Barstool Sports, who had written he was upset at the Mets first baseman’s lobbying the umpires to try to get the game in the previous night amid the rain. After the Cardinals tied it off Diaz, the game went into a delay and was eventually postponed until Friday.

“Are you kidding me? Why are you mad at me for having confidence in my teammates?” Alonso tweeted. “I’m a damn competitor and I’ll take my team over any one else any day. A real Mets fan doesn’t talk like that. Clean it up. #YaGottaBelieve.”

The bullpen is making it more challenging by the day.