The Mets’ nightmarish first half ended in typically ugly fashion, with another one-run loss, this one ending with Starling Marte striking out with the bases loaded in the bottom of the ninth.

The 3-2 loss to the Brewers left the Mets 36-45.

They’ve dropped four of five — and 18 of 24 — and are a season-worst nine games under .500, 17 games back in the NL East and nine back in the wild-card race.

They had a chance against Milwaukee closer Devin Williams in the ninth after Mark Canha walked with one out and Francisco Alvarez singled.

Brett Baty had already been removed from the game for pinch-hitter Danny Mendick in the seventh, so Buck Showalter had to go to light-hitting Luis Guillorme in the ninth.

Canha stole third, and Guillorme then grounded to first, moving Alvarez to second.

Brandon Nimmo walked to load the bases, but Marte whiffed to end it.

“It’s frustrating,’’ Buck Showalter said. “There’s a game there to be won. We’re snakebit from that standpoint. I keep hoping the worm’s gonna change.”

It’s getting late for that to happen, and they lost another opportunity to get headed in the right direction with Max Scherzer on the mound.

Back-to-back homers by Baty and Nimmo in the third gave Scherzer a 2-0 lead, but the right-hander gave up a two-run shot to Victor Caratini in the sixth to tie the game.

Showalter went to TJ McFarland in the seventh and the lefty allowed two of the three batters he faced to reach base before Dominic Leone entered and gave up an infield hit to pinch-hitter Owen Miller before Brian Anderson’s sacrifice fly gave Milwaukee the lead for good.

The Mets weren’t done teasing the fans at Citi Field, though, as Canha, pinch hitting for Daniel Vogelbach to start the bottom of the inning, singled to center against lefty Hoby Milner.

Alvarez flied out and Mendick, batting for Baty, hit a potential double-play ball to third, but Anderson booted it and the Mets had runners on the corners for Nimmo, who was hit to load the bases.

Marte grounded into an inning-ending double play, drawing boos from the crowd.

The decision to lift Baty against Milner was made because, as Showalter said, Milner is especially tough on left-handed hitters and the Mets, Showalter said, “were running out of outs” in the seventh.

Showalter said Baty would get more at-bats against lefties as he gets more experience, but the Mets are right now trying to develop players and win games at the same time.

It hasn’t worked.

After Scherzer cruised through the top of the first, he allowed consecutive one-out singles to Anderson and Caritini in the second to put runners on the corners.

But he got Brice Turang on a liner to short and Blake Perkins on a deep fly to left to strand two.

With the Mets in a tailspin, even when they catch a break, they have a hard time taking advantage of it.

Pete Alonso led off the bottom of the second with a pop-up down the right-field line. Turang, the second baseman, dropped his running attempt at a catch, but Alonso was overly aggressive and was thrown out at second from right field by Perkins.

Later in the inning, Jeff McNeil’s two-out double to left was wasted when Vogelbach was robbed of a run-scoring single by Turang, who made a diving play to his right and a strong throw to first.

All of that came back to get the Mets, who haven’t won a series since sweeping the Phillies in early June.

“When things are going really bad or tough or whatever we want to call it that’s going on right now, you want to look from a distance and see things that are wrong,’’ Francisco Lindor said. “You’ve got to stay in the moment and it’s tough, at times, not to look ahead. … It gets challenging because we’ve been losing a lot.”