The actress announced in October that she will be stepping down from the role of Rosie Webster, reportedly to start a new life in Wales with footballer boyfriend Scott Sinclair.
Flanagan has now elaborated on her decision to leave Corrie, claiming that she "instinctively" knew to quit once she turned 21.
"I didn't feel I would have grown as a person if I stayed," she told Hello. "I'd always felt that I would leave by the time I was 21, but when I made up my mind to go, it wasn't even as if I had a choice - I just knew instinctively that this is what I had to do.
"I've played the character of Rosie for 12 years, but now I just want to be Helen. It's time for me to be myself. I didn't enjoy playing her when she went through a promiscuous phase, though. Luckily she shows more heart these days.
"I realised I had to do this after taking a three-month sabbatical from April to June this year. During my time away from the Street, I felt free. One of the first things I did was cut my hair short and I dyed in bright blonde. I felt fearless and girlie. It was something I had wanted to do for a long time."
Flanagan previously denied reports that her anxiety attacks on the Corrie set sparked her desire to quit.
Rosie's departure from Coronation Street will air in February 2012.