Rock star Cisco Adler dumped gorgeous Mischa Barton because he was fed up with her mood swings and wild partying.

That's the REAL story behind the recent split of the sexy former "O.C." star and the Whitestarr singer, say sources.

When nude photos of hunky Cisco - taken by Paris Hilton - surfaced last month, the Hollywood grapevine jumped to the conclusion that 21-year-old Mischa dropped him.

"But Mischa didn't care about the photos," a source told The ENQUIRER. "She knew Cisco and Paris once had a relationship. Mischa is crazy about Cisco, she forgave him. Cisco broke up with her. She's just too much of a partier. He tried to get her to tone it down and even talked to her friends about reining her in. But nothing worked - even after her sister went into rehab."

Mischa's 19-year-old sister Hania, an aspiring singer, checked herself into rehab after "getting out of control the past six months," said the source.

"Cisco hoped Hania's problem would be a wakeup call to Mischa - but it hasn't been.

"Plus, Mischa's been down since producers killed off her character on "The O.C." and the show was canceled.

"She's had a lot of time on her hands and has filled it with late-night Hollywood parties. It was finally too much even for Cisco to handle - and now Mischa is devastated."

The lovestruck actress had been pestering the 28-year-old son of music mogul Lou Adler for a commitment, say insiders.

"Cisco gave her a diamond 'promise' ring, but Mischa was still pushing him for a formal engagement," according to the source.

Mischa's spokesperson confirmed that the two "decided to part ways" and explained that "they both have very demanding schedules."

But the source says Cisco told friends: "We're better off apart. She's too crazy for me!"