He revealed news of the split to The Daily Beast while at the Samsung Hope for Children gala Tuesday night.
"My greatest mistake right now is, I've been clinging to my art," he said. "In that, I have victory for my art and a great loss for my heart. At the moment, my beautiful fiancée is no longer my beautiful fiancée."
Mraz proposed to Prettyman in December, tweeting the happy news, "SHE SAID YES" on Christmas Eve.
No wedding plans had been decided, primarily because the couple decided to wait to tie the knot until marriage was legal for everyone.
"We can't get married until [gay] marriage is legal and equal," Mraz told Us Weekly in January. "I think giving people the right to marry will be a huge movement in civil rights."
But now, Mraz is just trying to come to terms with his recent split.
"Two halves don't make a whole," he continued Tuesday about his breakup with Prettyman. "Two wholes make a whole. In my relationship, I was giving myself away to make the relationship better, but in actuality, wasn't doing better by doing that. I became less of a man."
But just because they aren't in a relationship doesn't mean Mraz and Prettyman have cut each other out of their lives.
"We are still super friends, we go to yoga together, we surf together," he said. "We acknowledge the journey that each of us is on. We certainly want each of us to feel whole and complete. And it's when you're whole and complete that that attraction exists and it really thrives."
Watch Jason Mraz's 'The Remedy' video.