Billy Joel entered rehab in 2005 after being given an "ultimatum" by his then-wife

In the new HBO documentary Billy Joel: And So It Goes, the musician shares: "With the rehab, you don't go for somebody else. You have to go for yourself. You have to want to do it. I didn't want to do it."

Katie - who was married to Billy between 2004 and 2009 - noted that Billy struggled with addiction after taking a break from the spotlight.

She explained: "I felt like he needed to be creating, he needed to be making music, performing, to turn down that anxiety of not having an artistic outlet. And there were struggles with addiction. It was really hard to navigate that, because I had no experience with it."

Katie recalled there being a "fragile time" in their relationship after Billy returned home from rehab.

She reflected: "In a lot of ways it was hard to recover from that.

"I don't think either of us wanted it to not work out, but it just became obvious that it wasn't working."

Meanwhile, Billy recently admitted that he was hesitant about committing to his HBO documentary.

The music icon revealed that he was reluctant to reflect on some of the "stupid stuff" he's done in the past.

He told People: "I've resisted this kind of thing for so long. I'm sick of talking about myself. Some of the stupid stuff I did, that's painful to talk about. (But) they asked me for some thematic guidance. I said, 'Just tell the truth.'"

The chart-topping star initially just wanted to get the documentary "over with" because he doesn't enjoy talking about himself.

The Uptown Girl hitmaker said: "My goal was to get it over with.

"When I do interviews, people just ask you about yourself and you get a little self-conscious about it eventually. It's almost embarrassing.

"When you're talking about your personal life detached from the material ... I suppose there's a little bit of wariness involved."