Published Wednesday, 26 September 2012
The star, who serves as a judge on The X Factor, has opened up about the January separation for the first time in her new autobiography, Honest: My Story So Far - and reveals the end of their romance was rocky.
Contostavlos admits she began to worry Fazer, real name Richard Rawson, was seeing other women after he started staying out late at night, and she regularly demanded to check his phone for flirty messages.
In the book, serialised in The Sun, she recalls: "He started going out more while I was working and coming home later and later - I didn't have a clue where he was most of the time. We began arguing a lot.
"I found myself more jealous than I have been in any other relationship. 'Phone check' I'd randomly and regularly demand, holding out my hand. Fazer would be expected to hand me his phone so I could scan it for texts or messages from his ex or anyone else I might not approve of."
The singer pretended to be her boyfriend and messaged another woman in her bid to discover the truth: "I was texting stuff like: 'Ah, come on babe, you know I've got a girlfriend.' Then she'd text back: 'Well, that never stopped you before, did it?'"
Contostavlos attempted to patch up the relationship on a holiday to the Maldives in January, but the pair decided to part ways when they returned to the UK.
She adds: "I came to the conclusion I wanted out. I just couldn't take the stress anymore."