The couple initially separated last year - shortly following the birth of their son Tripp in December 2008 - but announced that they had rekindled their romance and were engaged again last month.
However, speaking to People, the daughter of former Alaskan governor Sarah Palin said: "It's over. I broke up with him."
The 19-year-old went on to claim that trouble in their relationship surfaced in mid-July when Johnston informed her of the possibility he had fathered a child with another woman. Palin declined to identify the woman in question although Johnston's ex-girlfriend Lanesia Garcia recently dismissed speculation that she is expecting a child with him.
Palin continued to say that the "final straw" was Johnston's alleged "obsession with the limelight", claiming: "[He flew] to Hollywood for what he told me was to see some hunting show but come to find out it was that music video mocking my family. He's just obsessed with the limelight and I got played."
She added that she has only seen Johnston once in the past three weeks and still hopes that she can find a solid relationship for herself and Tripp, 19 months, in the future.
A spokesperson for Johnston has yet to comment, although his attorney Rex Butler is quoted as telling the Washington Post that the couple might still reconcile.
"These young folk have a lot of pressure on them right now," he said. "All you have to do is watch that show Bridezillas - it's on and off, on and off, depending on the pressures."