Even when the group was brought back together for TV series The Big Reunion, he struggled.
During rehearsals, Sean was seen walking out before breaking down in a stairwell. He told the cameras: "In rehearsals, everything I felt in the past just came back.
"I suppose it's like...crying a lot of the pain from the past and the old Sean...it's crying it out."
Since then, the boy band has been on tour with their fellow Big Reunion bands - Atomic Kitten, 911, B*Witched, The Honeyz, Liberty X and Blue - and played at Virgin Media's V Festival.
Sean and fellow 5ive stars Abz Love, Scott Robinson and Ritchie Neville are also going on tour themselves in November.
Thankfully, when we caught up with Sean at V Festival, he was in good spirits and insisted he is now "fine" after admitting his emotional struggles on the TV show.
The singer, 32, told Reveal: "I'm absolutely fine. I'm just a normal human being who has his ups and downs and I just show how I feel.
"That's what The Big Reunion was about. It was all about telling the truth."
Abz added: "It's always being the case with us. We've literally had a fight on stage - it's probably not the best thing to do, but we tell it how it is. If we're p*ssed off, we say it."
5ive will return to our screens tomorrow night (5 September) in The Big Reunion On Tour, on ITV2 at 9pm.
In the first episode of the three-part mini series, former 5ive star Jason 'J' Brown will make an appearance and hit back at claims he was a "bully" during the 90s.
We can't wait!
Check out 5ive in this throwback video for 'When The Lights Go Out' below: