NEW YORK >> Terry Collins knew what he wanted to do. And he didn’t do it.

The Mets manager decided to pull Matt Harvey with a 2-0 lead after eight innings, with New York three outs from a victory that would force the World Series back to Kansas City. Closer Jeurys Familia was coming in.

Harvey argued. Collins gave in.

“Obviously I let my heart get in the way of my gut,” Collins said early Monday after a crushing season-ending, 12-inning 7-2 defeat. “I love my players. And I trust them. And so I said, ‘Go get ’em out.'”

It wasn’t to be, and now Collins will spend the rest of the offseason, perhaps the rest of his life, getting second guessed.

“I won’t be sleeping much the next couple of days, I’ll tell you that,” he said.

At 66, Collins is the oldest manager in the major leagues. He aged incalculably during nine pitches over roughly four minutes that will live in Mets’ infamy.

Pitching coach Dan Warthen had told Harvey he was done after eight innings of four-hit ball and 102 pitches. Harvey, who had thrown nine pitches in each of the previous two innings, said “No way” and headed to the other end of the dugout. The bearded, 6-foot-4 pitcher towered over his 5-foot-9 manager as he stated his case.

“I want this game. I want it bad. You’ve got to leave me in,” Collins remembered Harvey saying.

“Matt, you’ve got us exactly where we wanted to get you,” the manager replied.

“I want this game in the worst way,” Collins said Harvey told him.

And Collins relented, giving Harvey a chance to go for the second complete game of his big league career.

“When you looked in this kid’s eyes, when he came off that inning,” Collins said. “He’s been through a tough summer. He’s been beaten down, and I just trusted him. I said, ‘You got it. You’ve earned this. So go get ’em.’ So it’s my fault. It’s not his.”

A supercharged Harvey sprinted from the dugout to the mound, then walked Lorenzo Cain on a full-count slider.

“If you’re going to let him just face one guy, you shouldn’t have sent him out there,” Collins said.

Cain stole second on the next pitch, and Eric Hosmer sliced a fastball into the left-field corner, pulling the Royals within a run.

Collins walked out to the mound, took the ball and handed it to Familia.

“When the double hit, that’s when I said I’ve got to see if we can get out of this with only one run,” Collins said. “And it didn’t work. It was my fault.”

The relentless Royals tied the score when Mike Moustakas advanced Hosmer with a groundout, and Salvador Perez hit a grounder to third; Hosmer broke for home as David Wright threw to first and slid in headfirst ahead of Lucas Duda’s wayward throw.

“I think there’s a lot of fingers that you can point at, but I wanted the ball, and in that situation I did what I could to go back out there,” Harvey said. “In a different world it would have been a lot different.”

Collins has done outstanding work with the Mets, keeping players motivated during four losing seasons before this year’s surprising run to the NL pennant. He has talked about how getting questioned is part of a manager’s job description and said during the League Championship Series that when a decision doesn’t work, “my wife on the way home tells me it was a dumb move.”

Lauded for a string of successful — and sometimes unconventional — moves during the first two rounds, Collins was the toast of the town.

But that was then.

He already was being questioned by his fans for his handling of the bullpen in Game 4, when the Mets wasted an eighth-inning lead in a 5-3 loss. By using Familia for the last three outs of Friday’s 9-3 Mets’ blowout, Collins didn’t have his closer available for a six-out save Saturday. A pair of one-out walks by Tyler Clippard led to Familia’s second blown save, helped in part by second baseman Daniel Murphy allowing a slow grounder under his glove for an error that let in the tying run.

Familia escaped further trouble in the ninth Sunday and pitched a 1-2-3 10th, but Collins pinch hit for him in the bottom half. Jonathon Niese survived a one-hit 11th, and the Royals broke through in the 12th against Addison Reed, who allowed pinch-hitter Christian Colon’s RBI single, and Bartolo Colon, who gave up Cain’s three-run double.

New York could have won this Series in five games. The Mets led in every game and at the end of 24 of the 53 innings, trailing at the conclusion of only 13.

If only Familia hadn’t given up Alex Gordon’s tying homer with one out in the ninth inning of the opener.

If only Murphy’s error hadn’t allowed the tying run to score with one out in the eighth inning of Game 4.

If only the Mets had held the lead with two outs to go in Game 5.

“Sometimes you let your heart dictate your mind,” Collins explained. “We had said going in if Matt gave us seven, Jeurys was going to pitch two. I’ve got one of the best closers in the game. I got him in the game, but it was a little late. And that’s inexcusable for me.”