Jessica Simpson

Jessica Simpson & John Mayer Split – Again

06/05/2007 at 05:15 PM EDT

Jessica Simpson and John Mayer have ended their romance – again, and this time for good. Possibly.

"It has been rocky and it had been rocky, and it reached the end of the road," a source close to Mayer tells PEOPLE. "It is definitely over – the relationship has ended."

Those on Simpson's side confirm that the romance has come to an end, but say the door is always open for the pair to reunite.

"They broke up Sunday night, but who knows what the future will hold," says a Simpson source. "This is the twelfth time they've broken up. Their relationship is volatile. Last week they felt better than ever. This week, things are rocky. Who knows what the future will bring. "

Simpson, 26, and Mayer, 29, have been together about a year, though were coy about the relationship until recently.

Just last month, the couple split, but then only days later appeared to have reconciled after a heavy round of emailing and talking.

"They tried to make it work," says the Mayer source, who adds that the couple's time together in New York and more recently in Cabo San Lucas, Mexico were an attempt to talk things over.

But that was then. Now, says the source: "It's over."

Simpson's pal says the singer is "used to" the back and forth with Mayer by now. "She loves [John] but she has to be true to herself."

As for any reason's behind this most recent split: "Every relationship has its differences, but with Jessica and John those differences were starting to become more and more apparent," says the Mayer source.

• Reporting by KC BAKER