NEW YORK — Bryant Jennings was always very tough on himself. The Philadelphia heavyweight would nitpick and probe, guess, then second-guess himself. He was a perfectionist. He admitted it.

These past few years have forced him to change some of his ways of thinking. One is learning to ease up on himself, to relax and be patient.

Jennings finished 2017 with the intention of banging some rust loose after being stagnant for two years. He closed ’17 with a pretty impressive third-round TKO of Donnie Haynesworth on the first fight of the Vasyl Lomachenko-Guillermo Rigondeaux undercard at the Madison Square Garden Theater Saturday night.

“I wouldn’t give myself a grade for this,” Jennings said. “I trust my strength and conditioning. I was happy I got some rounds in. Even though it was three, I wanted to see and feel (Haynesworth’s) power. I’m an attacker, but I have to see what I’m working with. He was 271 and I was 227¼ for this. I had to get used to moving around, and get used to my distance.

“It was a right that landed on the chin that stopped him. I have to trust in my abilities and figure out what I have in front of me.”

Jennings’ loss to Luis Ortiz still sticks with him. Ortiz fought Friday night, wiping out the same Daniel Martz in two rounds that Jennings beat in his comeback fight with a second-round TKO back in August.

“That loss does still stick with me, but believe it or not, I accepted that,” Jennings said. “Upon all of the other stuff going on with Ortiz, that helped me deal with it. The past is the past. I would love another shot at Ortiz, but he has to be clean. That loss taught me a lot. I’m more grounded.

“I’m clocking in and I am more relaxed. But I also take boxing a lot more seriously. Let them try these heavyweights come and get me. I made a mistake in the Ortiz fight. I went straight in and everyone knows that’s not my fight. I’ve learned to adjust and feel comfortable.”

Jennings (21-2, 12 knockouts) got a bit of a workout in the first against Haynesworth (13-2-1, 11 KOs); with his jiggly middle, Haynesworth had the Philly heavyweight backing up and defensive. The second round, however, saw a little change. Jennings fired a jab at Haynesworth’s ample midsection. He then crowded the larger Greensboro, North Carolina, fighter and burrowed into him, firing shots to his body and head.

To his credit, Haynesworth kept fighting back when the tide of the fight had swayed to Jennings. It wasn’t long before “By-By” snapped the short right off Haynesworth’s chin and wobbled him. Haynesworth teetered and referee Arthur Mercante Jr. stepped in and waved it over at 2:29.

“I’m happy with the outcome,” said John David Jackson, Jennings’ trainer. “Bryant doesn’t have much of an amateur background and it takes him some time to get started. I think if I can get one more from Bryant they’re talking bigger fights for him. I liked when Bryant lands his right hand.

“I constantly tell him that he’s the strongest man in boxing. He just doesn’t realize it. When he let go of his right hand, he showed that. I think he needs to get more relaxed and go to the body better. That will open up the right hand for him. Bryant is willing to learn. I’m happy with the win.”