Gwen Stefani went through months of "torture" during her separation from ex-husband Gavin Rossdale.

The No Doubt frontwoman split from the British singer last summer (15), after 13 years of marriage, amid allegations of Gavin's infidelity. Their divorce was finalised in April (16).

Gwen has never revealed the specific reason behind the split, but she admits it was a difficult time for her.

"Nobody except for my parents, the people involved, and whoever they told (know what happened between us)," she tells Cosmopolitan magazine. "Nobody would believe it if I could really say what happened. I went through months and months of torture."

The singer wouldn't reveal if she knew there were issues before they separated, but explains she put her heartache into her music.

"All you have to do is listen to every single song I've ever written and it's all there," she says. "It's an ongoing theme that I've had for a long time."

She has since found love with her fellow The Voice mentor Blake Shelton, who divorced singer Miranda Lambert in July, 2015, and the couple has already shared experiences Gwen didn't have with Gavin - including writing music together.

"We were both having trust issues, as you would if you were us," she continues. "It was like, 'I have so many problems. How can I now be getting into another potentially huge problem? What am I doing?'

"Blake wrote (song) Go Ahead and Break My Heart and sent it to me. He was like, 'Help me finish it'. I wrote a verse and sent it to him, but he was gone because he doesn't live in Los Angeles. He didn't have wifi, so he wasn't answering me back! As soon as he came back to town, he brought his guitar over. We were like, 'We wrote a song together! This is crazy!'"