Have you ever said “I do” when deep down you really meant “I don’t”? Did you say “yes”even though you knew the guy popping the question was Mr. Wrong?

Even if you haven’t, chances are you know someone who has. “Nightline” sat down for an interview with Anne Milford and Jennifer Gauvain, the authors of “How Not to Marry the Wrong Guy,” to talk about women in serious relationships who struggle with the decision to take the plunge into an unhappy marriage or become a runaway bride.

Milford canceled her own wedding five months before the big day, and says that she has never regretted her decision. Gauvain is a licensed clinical social worker whose research focuses on couples and families. After interviewing hundreds of divorced women for their book, the two said that roughly 30 percent knew they were marrying the wrong guy before their wedding day, but didn’t know how to stop it.

“It could be a $10 million wedding or a $10,000 wedding, it’s still the same pressure externally of ‘I have to do this’,” said Gauvain.

“[Brides-to-be] think it’s too late to call it off, they don’t want to let anybody down,” Milford added.  ”And we always encourage women to remember that if you’re having these serious doubts … you need to focus on cancelling the relationship.”

Milford and Gauvain’s self-help book examines all angles of serious relationships, including uncomfortable conversations, sticky situations and ultimately, how to recognize you are with the wrong guy and break-up with him. Here is a sampling of tips from their book, which is in stores now.

Tips for How to Break-Up With Mr. Wrong, Even If He’s Your Fiancé:

1. Pay attention to your gut feeling and don’t ignore the red flags (see below).
2. Find the courage to take action and cancel the wedding. Often times women feel too guilty or shameful to speak up.
3. The break-up conversation will be difficult but it doesn’t need to go on for hours. Prepare what you are going to say beforehand.
4. Talk to someone you trust before you do it to help you choose your words carefully.

Common Red Flags The Authors Say Other Brides Ignored:

-          Avoidance: He avoids your questions, company or conversations about your relationship.

-          Dependence: Does he have any friends?

-          Extreme differences: His personality clashes with yours and causes problems.

-          Irresponsibility: He can’t be counted on for help, especially financially.

-          Family issues: He has a strained – or strange – relationship with his family.

-          Meanness: He is disrespectful and inconsiderate.

Why Do Some Women Go Through With a Bad Marriage Anyway?

1. Age: The fear of being too old to get married or have children if they wait.
2. Hurt feelings: “I don’t want to hurt him, he needs me.”
3. He’ll change: “He was great when we started dating.”
4. Settling: “All the good guys are taken.”
5. Divorce is seen as serious option: “If it doesn’t work out, I’ll just get divorced.”
6. Financial fears.

For more on Milford and Gauvin’s findings and suggestions, read “How Not to Marry the Wrong Guy.” Tune into “ Nightline” tonight at 11: 35 p.m. ET to watch the full interview and hear stories from other brides who regret going through with their wedding.