The 34-year-old supermodel - who threw a joint party with the rocker to 'celebrate' the end of their six year marriage in June 2011 - admits the breakdown of their union was an emotional time, but she knew she had to be ''grown-up'' about the situation.
She said: ''It's awful. I'm not saying it isn't. There were times when Jack and I felt, 'What is happening here?' It was emotional. But it's one of those moments in your life when you realise that you are a grown-up.''
Karen felt she had a ''responsibility'' to the couple's children, Scarlett, six, and Henry, five, to remain friends with the 'Steady As She Goes' hitmaker following their separation.
She explained to the May edition of Britain's Harper's Bazaar magazine: ''You have a responsibility to your children. It's not like splitting up with a boyfriend in your twenties. You are breaking up with a whole life. It's not an easy process. But you've got to be friends, there is no choice. I am the product of a divorced family so I felt a responsibility to make that happen.''
And despite the setbacks in her personal life, the Scottish beauty is looking forward to the future.
She added: ''I've overcome so much, shaken off so much self-doubt. I'm like, 'God, my life is just starting now. I have full faith in the second half of my life.''