Heidi Klum was unhappy for "some time" before ending her relationship with Martin Kirsten.
The supermodel split from her former bodyguard after a year-and-a-half of dating a few weeks ago.
Although Heidi realised she wasn't in love with Martin, friends say she found it hard to leave him because he was so close to her four children, Leni, nine, Henry, eight, Johan, seven, and Lou, four.
A source told In Touch Weekly magazine: "Heidi has been very unhappy for some time now. He was so entrenched in Heidi's life, her kids absolutely adore him, it was just impossible for her to make a clean break."
Heidi, 40, got together with Martin, 41, only a few months after she split from her husband of seven years, singer Seal, in January 2012 and friends claim despite her deep bond with the security guard, she was on the rebound from the musician.
The insider added: "She loves Martin but she isn't in love with him anymore. He was just a convenient rebound."
When the couple initially got together, Heidi said of Martin: "He's been helping me for four years now, and he takes wonderful care of the kids. He plays with them, he protects them. I trust him."
He was also on hand when to help saved Henry and two nannies from a riptide in Hawaii which could have drowned them in early 2013.