Speaking to Hello! magazine, the 63-year-old explained that he'd popped the question to Hayley, 36, during a romantic beachside picnic in Malibu.
'I thought he was just joking around at first. He's asked me before, but it was never for real,' Hayley told the mag.
'Then he pulled out the ring. I cried so hard that he didn't get around to actually asking, 'Will you marry me?' until about 10 minutes later.'
'I genuinely had no idea he was going to do that. I'm still so overwhelmed.'
David, who has had two previous marriages, to singer Catherine Hickland and actress Pamela Bach, and has two grown-up daughters, added, 'I had a saying, 'You don't get married because you want to live with someone, you get married because you can't live without them'.
'I felt that way. I knew that I was really unhappy without her. I waited to propose because I thought I was too old for her.'
Commenting on the 27-year age gap between the two, Hayley stated that she didn't want things that might happen in the future to spoil the couple's current happiness.
'I don't want to miss out on being with someone I love because of what might happen down the line,' she said.
David and Hayley met in 2011, when she asked him for a picture on the set of Britain's Got Talent. He said that she could have the snap in exchange for her number, and they had their first date shortly after.
At the time, Hayley had been working in her local department store in Wales, and living at home with her parents and sister. She had been single for six years before meeting David.
'He has completely changed my life,' she revealed to The Sun in 2012. 'I couldn’t have dreamt this up. And all because I asked him to pose for a picture with my sister and me and we ended up falling in love.'