The Girls Aloud singer has for some time been known for her love of the party lifestyle but revealed the boozing got out of hand after the break-up of her relationship with then-fiancé Tom Crane.
Amidst rumours of rowing about her partying, Sarah and Tom called off their engagement in September.
Soon afterwards, Harding checked into rehab in search of treatment for depression and alcohol addiction.
Speaking to Hello! magazine, Sarah explained, "I was so down but I just tried to keep busy and, to numb myself, I was drinking more. It was my way of escaping.
"One minute I was deliriously happy, to the point of hysteria, and totally in denial about my problems saying, 'I'm fine, I'm fine'. The next it hit me - and when it did, it hit me hard."
The tipping point came when Sarah's Girls Aloud pals Kimberley Walsh and Nicola Roberts invited her for dinner the day after a heavy fashion night out.
"I wasn't in a good state at all," Harding told the mag. "I was very hungover. But the first thing I wanted to do was open a bottle of wine. I know they were really worried about me."
She went on, "The day I left for rehab, I was such a mess... I was crying hysterically. I felt ashamed, embarrassed, terrified - my emotions were all over the place."
With a stint in rehab under her belt, Sarah hopes to make a fresh start, and added, "I've turned 30 now and, although there will always be a bit of the rock chick in me, I'm trying to embrace my softer and more feminine side."
We wish her all the best.