The Fun. and Bleachers musician called it quits with "Girls" creator and star Lena in January, and just days later bloggers and gossips started suggesting he had already moved on to date Lorde, real name Ella Yelich-O'Connor.
He shut down the unfounded speculation in a rare Twitter post addressing his personal life, and Lorde also insisted there was no truth to the rumours.
Lena has not spoken to Lorde since she her relationship with Jack ended, but she insists she believes him. However, she admits it was sill difficult for her to hear.
"It was so embarrassing," she tells The Cut. "It was awful because I felt, like, a weird... I don't think anything happened between them. I can never know someone else's life. I have never spoken to Ella about it. We haven't talked since Jack and I broke up. It was awful, and I couldn't do anything about it except trust that what he was saying to me was true".
The 32 year old also reveals she has a difficult time looking at Antonoff's new girlfriend Carlotta Kohl's Instagram pictures.
"I thought I was kind of proving weird girls can have love too," she says. "And now he's dating somebody who looks regular and normal and like girls are supposed to look."
However, she insists it was best for them to move on from each other because they weren't confronting their issues. She explains her battles with endometriosis and fibromyalgia caused them to ignore their problems.
"Our relationship probably lasted longer than it should have," she continues. "He's a very loyal person, so he was not going to bail when the going got tough... But when you're sick, so much energy goes into making sure the other person is well that you're not even noticing maybe our schedules aren't compatible. Maybe we want different things out of our lives. Maybe we have different attitudes about what family means. Maybe these essential questions that people have to ask themselves all the time are not being asked, because we just want to make sure that I don't pass out at the grocery store."