EXCLUSIVE: J.Lo on Affleck's Proposal

July 10 -- When Jennifer Lopez went to visit her beau Ben Affleck's family in Boston, he promised her it would be a relaxed occasion, with a messy house and maybe some pizza.

On their way there, he was a little nervous, although the singer-actress, whose romance with the handsome leading man has been public since last summer, didn't suspect anything. But when the couple got to the house and Affleck opened the door, she had a surprise. "It's just a blanket, a quilt of rose petals, all over the whole entire house," she remembers. "So many candles, and vases, bouquets. And my song 'Glad' was playing ... I walk in and I was just like overwhelmed. I wasn't expecting it, and I was just like 'Oh my God.'"

Then she asked who had done it all, and Affleck told her, "My mom helped me do it." Then, says Lopez, "I just started sobbing. Crying. I was like 'Oh! My God!'"

As tears streamed down her cheek, Affleck read a letter he had written saying all the reasons he loved her and wanted to marry her. "I had cried a lot over sadness over the years. And for the first time in my life, I cried incredible purging tears of happiness," Lopez told Primetime's Diane Sawyer. "It was the most cleansing feeling and the most wonderful feeling I had ever had."