Sarah Ferguson, the Duchess of York, said not being invited to Prince William and Kate Middleton's wedding was "so difficult" for her to cope with.
In an television interview with chat show host Oprah Winfrey, the duchess revealed she spent April 29 in Thailand.
Her ex-husband the Duke of York, who accompanied their daughters Beatrice, 22, and Eugenie, 21, to Westminster Abbey, made her feel part of the occasion by talking to her on the telephone.
Asked how hurt she felt by the snub, the 51-year-old former royal replied: "It was so difficult. Because I wanted to be there with my girls ... and to be getting them dressed and to go as a family.
"And it was also hard because the last bride up that aisle was me."
Sarah, who was filmed offering an undercover reporter access to the Duke of York for £500,000 last year, said she did not feel ostracised by the lack of an invitation.
But she added: "I felt that I ostracised myself by my behaviour, by the past, by living with all the regrets of my mistakes, that I sort of wore a hair shirt and beat myself up most of the day thinking and regretting: why did I make such a mistake?
"Why have I made so many mistakes? So I did spend a good three hours on that."
In the interview, to be broadcast in the US today, she said that while in Camelia, Thailand, on the day of the wedding, she spent "all morning" talking to Andrew.
"He was saying, 'It's okay. Just remember we had such a good day. Our wedding was so perfect'. Because we're such a unit together. He made me feel very part of the day on April 29th."
When asked if she felt rejected, she replied: "I think what I felt was that I went through the phase of feeling so totally worthless and that (it) was quite right they didn't invite me."
Sarah said she believed William's mother would have been "so proud of her son", and added: "I really love the feeling that sort of Diana and I both weren't there.
"But I'm here to say how proud she would have been and Kate looked utterly beautiful."
Promotional clips on the Oprah Winfrey website claim that in the interview, filmed in Chicago, the duchess's daughters will "break their silence". The host also asks about her close relationship with her former husband, saying: "Why don't you get married again?"
The duchess is to have a show on Winfrey's new cable station, OWN.