The pair met when they filmed thriller movie Three together in 2003. Brook had previously been in a long-term relationship with actor Jason Statham.
"I fell in love with Billy and felt like I was under a spell. They say love is an illness and I do believe that," Kelly explained. "I became lovesick.
"When you fall in love, life's problems seem to disappear. That is exactly what happened. It's like a chemical imbalance in your body, I suppose."
Brook split from Zane temporarily in April this year after their differing attitudes towards marriage became clear. However, they reunited just weeks later.
Asked about her partner's faults, Kelly replied: "He has a sock issue. They are everywhere. We have people round for dinner and they pull out a random sock stuffed down the side of a sofa. How did that get there? How does he do it? Does he only wear one sock at a time?"