Justin Theroux on the cringeworthy way his last high school relationship ended (Exclusive)

High school to college relationships can be messy, even for Justin Theroux.

On Saturday, the 46-year-old actor dished to ET's Nischelle Turner in New York, at a roundtable discussion with the cast of The Spy Who Dumped Me, about his worst breakup ever -- and it wasn't his recent split from Jennifer Aniston. In the film, Theroux plays Mila Kunis' ex-boyfriend, who turns out to actually be a CIA agent.

The cast was asked about their worst breakup stories and Theroux, 46, had a doozy.

Back when he was about 17, he said he had a pretty clingy girlfriend. 

"I had a girl in high school who did that extra crazy thing where she … like, I was going to college and she had to stay here and she was like, 'I miss you, I love you, why aren’t we together' and I was like, '[I'm] going to work and trying to get ready for college.'"

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Justin Theroux out and about
NEW YORK, NY - AUGUST 14: Justin Theroux attends the INTERSECT by Lexus Preview Event on August 14, 2018 in New York City. (Photo by Jamie McCarthy/Getty Images for INTERSECT by Lexus )
LOS ANGELES, CA - JULY 25: Justin Theroux attends the premiere of Lionsgate's 'The Spy Who Dumped Me' at Fox Village Theater on July 25, 2018 in Los Angeles, California. (Photo by Rich Fury/WireImage)
NEW YORK, NY - MAY 07: Justin Theroux attends the Heavenly Bodies: Fashion & The Catholic Imagination Costume Institute Gala at The Metropolitan Museum of Art on May 7, 2018 in New York City. (Photo by Jamie McCarthy/Getty Images)
NEW YORK, NY - MAY 07: Justin Theroux attends the Heavenly Bodies: Fashion & The Catholic Imagination Costume Institute Gala at The Metropolitan Museum of Art on May 7, 2018 in New York City. (Photo by Frazer Harrison/FilmMagic)
NEW YORK, NY - MAY 01: Actor Justin Theroux and Amy Sedaris are seen taking a cab in noho on May 1, 2018 in New York City. (Photo by Raymond Hall/GC Images)
NEW YORK, NY - APRIL 10: Actor Justin Theroux is seen biking in Soho on April 10, 2018 in New York City. (Photo by Raymond Hall/GC Images)
NEW YORK, NY - APRIL 02: Justin Theroux on April 2, 2018 in New York City. (Photo by Gotham/GC Images)
PARIS, FRANCE - MARCH 06: (L-R) Michelle Williams, Justin Theroux, Emma Stone and Louis Vuitton's executive vice president Delphine Arnault attend the Louis Vuitton show as part of the Paris Fashion Week Womenswear Fall/Winter 2018/2019 on March 6, 2018 in Paris, France. (Photo by Bertrand Rindoff Petroff/Getty Images)
PARIS, FRANCE - MARCH 06: Justin Theroux attends the Louis Vuitton show as part of the Paris Fashion Week Womenswear Fall/Winter 2018/2019 on March 6, 2018 in Paris, France. (Photo by Bertrand Rindoff Petroff/Getty Images)
PARIS, FRANCE - MARCH 05: Actor Justin Theroux is seen on March 5, 2018 in Paris, France. (Photo by Marc Piasecki/GC Images)
PARIS, FRANCE - MARCH 05: Actor Justin Theroux is seen on March 5, 2018 in Paris, France. (Photo by Marc Piasecki/GC Images)
NEW YORK, NY - FEBRUARY 12: Actor Justin Theroux is seen walking in Soho on February 12, 2018 in New York City. (Photo by Raymond Hall/GC Images)
NEW YORK, NY - FEBRUARY 08: Actor Justin Theroux attends the Adam Selman front row during New York Fashion Week: The Shows at Gallery I at Spring Studios on February 8, 2018 in New York City. (Photo by John Lamparski/Getty Images for New York Fashion Week: The Shows)
NEW YORK, NY - NOVEMBER 30: Justin Theroux attends an evening honoring Louis Vuitton and Nicolas Ghesquiere at Alice Tully Hall at Lincoln Center on November 30, 2017 in New York City. (Photo by Noam Galai/WireImage)
NEW YORK, NY - OCTOBER 25: Actor Justin Theroux is seen on October 25, 2017 in New York City. (Photo by UMV/Star Max/GC Images)
NEW YORK, NY - SEPTEMBER 19: Justin Theroux is seen on the set of 'Good Morning America' on September 19, 2017 in New York City. (Photo by Raymond Hall/GC Images)
NEW YORK, NY - SEPTEMBER 19: Actor Justin Theroux leaves the 'Good Morning America' taping at the ABC Times Square Studios on September 19, 2017 in New York City. (Photo by Ray Tamarra/GC Images)
WESTWOOD, CA - SEPTEMBER 16: Actor Justin Theroux attends the premiere of 'The LEGO Ninjago Movie' at Regency Village Theatre on September 16, 2017 in Westwood, California. (Photo by Jason LaVeris/FilmMagic)
WESTWOOD, CA - SEPTEMBER 16: Actor Justin Theroux attends the premiere of 'The LEGO Ninjago Movie' at Regency Village Theatre on September 16, 2017 in Westwood, California. (Photo by Jason LaVeris/FilmMagic)
LOS ANGELES, CA - SEPTEMBER 12: Justin Theroux is seen at LAX on September 12, 2017 in Los Angeles, California. (Photo by starzfly/Bauer-Griffin/GC Images)
LOS ANGELES, CA - JULY 27: Actors Jason Bateman (L) and Justin Theroux attend the 5th Annual Ping Pong 4 Purpose on July 27, 2017 in Los Angeles, California. (Photo by Allen Berezovsky/WireImage)
HOLLYWOOD, CA - JULY 26: Jennifer Aniston and Justin Theroux attend the ceremony honoring Jason Bateman with Star On The Hollywood Walk Of Fame on July 25, 2017 in Hollywood, California. (Photo by JB Lacroix/WireImage)
SAN DIEGO, CA - JULY 20: Actors Dave Franco (L) and Justin Theroux of 'Lego Ninjago' at 2017 WIRED Cafe at Comic Con, presented by AT&T Audience Network on July 20, 2017 in San Diego, California. (Photo by Phillip Faraone/Getty Images for WIRED)
SAN DIEGO, CA - JULY 21: (L-R) Actors Dave Franco, Kumail Nanjiani, Abbi Jacobson, Justin Theroux, Michael Pena, Olivia Munn and Zach Woods attend MT Fandom Fest at PETCO Park on July 21, 2017 in San Diego, California. (Photo by Kevin Winter/Getty Images for MTV)
NEW YORK, NY - JULY 18: Jennifer Aniston and Justin Theroux seen leaving a restaurant in West Village on July 18, 2017 in New York City. (Photo by Robert Kamau/GC Images)
NEW YORK - JULY 18: Justin Theroux in a sleeveless print tee is seen grocery shopping in West Village on July 18, 2017 in New York, New York. (Photo by Josiah Kamau/BuzzFoto via Getty Images)
NEW YORK, NY - JULY 18: Actor Justin Theroux is seen biking in Soho on July 18, 2017 in New York City. (Photo by Raymond Hall/GC Images)
LOS ANGELES, CA - JUNE 19: Actor Justin Theroux attends SAG-AFTRA Foundation's Conversations with 'The Leftovers' at SAG-AFTRA Foundation Screening Room on June 19, 2017 in Los Angeles, California. (Photo by Vincent Sandoval/Getty Images)
LOS ANGELES, CA - JUNE 19: Actor Justin Theroux attends SAG-AFTRA Foundation's Conversations with 'The Leftovers' at SAG-AFTRA Foundation Screening Room on June 19, 2017 in Los Angeles, California. (Photo by Vincent Sandoval/Getty Images)
LOS ANGELES, CA - JUNE 04: Justin Theroux, Kevin Garvey on stage at HBO's 'The Leftovers' - FYC on June 4, 2017 in Los Angeles, California. (Photo by FilmMagic/FilmMagic for HBO)
LOS ANGELES, CA - JUNE 04: Actors Carrie Coon, Nora Durst and Justin Theroux, Kevin Garvey attend HBO's 'The Leftovers' - FYC on June 4, 2017 in Los Angeles, California. (Photo by FilmMagic/FilmMagic for HBO)
NEW YORK, NEW YORK - JUNE 01: (L-R) Carrie Coon, Justin Theroux and Amy Brenneman attend the 'The Leftovers' FYC New York Screening at Metrograph on June 01, 2017 in New York City. (Photo by Mike Coppola/Getty Images)
NEW YORK, NY - MAY 30: Justin Theroux at Good Morning America on May 30, 2017 in New York City. (Photo by Gotham/GC Images)
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Kunis interjected, "I feel like that’s very sweet, I don’t understand the problem."

Oh, but there was a problem.

"It sounds very sweet, then I did the romantic thing of surprising her. I made the money for the summer to go to college and then I was like, I’ve got two weeks before I got to go, let’s go see what she’s up to, you know, like 'click click,' open the door," Theroux continued. "Her roommate was like, 'Oh my god, what’re you doing here?' and then I was like, 'Shhh' and I went into her room and someone was there."

That would be, his girlfriend, plus someone else.

"And then I realized, her roommate wasn’t doing the 'Oh my god, what’re you doing here?' she was actually like, 'Oh god, what’re you doing here?' I was totally misreading the situation."

Instead of a romantic surprise, he described a very different feeling upon opening that door and seeing what he saw.

"Like blood, blood, blood coming out of my ears," he said.

The Spy Who Dumped Me hits theaters August 3.

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