The 53-year-old didn't give any excuses but said he would have liked more time to turn things around at the club.
"I worked all my life to get my dream job and I failed," Kirwan said.
"It was horrible. I wasn't sleeping. I'd made a lot of mistakes but it was a five-year turnaround not a three-year turnaround."
Kirwan said the experience at the Blues took a huge toll on him but that it also helped him grow both personally and professionally.
"I've always taken the opportunities that have been presented and that hasn't always been easy. I've been scared at times, totally out of my depth. But all those experiences have helped me grow.
"The Blues was one of the best things to happen to me, it just didn't work out. It was a testing time for me, my mental health and my career. After that experience I took some time off to realise what I would do.
"I love the game. I still love the Blues. The game has given me everything, so how can you turn your back on a game that has given you everything and not still love it?"
Kirwan is still involved in the sport as a TV pundit and is a strong advocate for mental health and depression awareness.