Britney Spears reveals heartbreak over Kevin Federline

Britney Spears plans to expose some of the frustration she felt in her former marriage, PEOPLE reports.

A source tells PEOPLE that "one of the most personal songs" in Spears's upcoming album Blackout, to be released Oct. 30, was written by singer-producer Pharrell Williams .

Called "Why Should I Be Sad," the track makes references to money, cars - and other women. As Spears, 25, sings in one part of it: "It just seemed that Vegas/ Only brought the playa out of you."

Williams, 34, said : "At the end of the day she is a sweet person. She's just made a couple of decisions that none of us would necessarily agree with, but you try being that age and living under that kind of pressure. I wish her well. I pray for her. I hope she fares well throughout all of this craziness."

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Says a source about "Why Should I Be Sad": "The song is definitely about Britney and Kevin's relationship when they were married.