Katy Perry was ''very depressed'' after splitting from Russell Brand.

The 'Roar' singer parted ways with her ex-husband in December 2012 after just 14 months of marriage, and says she channelled her feelings from the heartbreak into penning 'By The Grace Of God', which features on her new album 'Prism'.

She told Entertainment Weekly: ''Well, imagine what you go through. Imagine what happens when you go through a break up. We all go through break ups and we all get very depressed and desperate. The lyrics are very exact and autobiographical.

''That's how I write. But the one thing about those lyrics is you can hear me finding my strength throughout the song. It starts off really low and then I kind of stand up for myself and say, 'No!' ''

The 28-year-old star - who has since moved on and found love again with John Mayer - had to force herself to snap out of her melancholic mood in the months following her separation from comedian Russell because she found herself wallowing in misery.

Katy said: ''Sometimes you look in the mirror when you're crying and if you look in the mirror it will make you cry more because you're feeling sorry for yourself. And then sometimes you look in the mirror and you cry and I've been like, 'Snap out of it! It's time to - come on - grow up! No!'

''There's almost like this inner warfare that comes out, this inner battle between the good angel and the bad devil.''