"Not that anyone should care, but in order to not give the press the chance to invent crazy and hurtful drama, I prefer to tell my fans personally that Nick and I have decided to be apart," the 41-year-old wrote.
"We have been having too many problems with figuring out how to spend time together and because of my work and now his, its been getting worse and worse. Not fun anymore," she continued. "We are still very close but we believe it's the best thing for us right now." (The pair were last seen together at the White House Correspondents' Dinner earlier this month.)
Vergara and businessman Loeb, 38, first began dating in 2010. They called it quits in May 2012, but reconciled and got engaged two months later while celebrating Vergara's 40th birthday at a Mexico resort.
Last year, the Colombian native talked about the possibility of having a child with Loeb, and the decision to freeze her eggs. "My doctor had told me, 'We have a small window.' I said, 'That's so rude!' I already have a kid, so I never thought I would need to do that," she said in an interview with Cosmopolitan. "But I will have to use a surrogate, because I had thyroid cancer and lots of radiation. I don't want to wait forever, but not yet, because I'm working like an animal."
Vergara has previously been linked to singer Craig David and actor Donal Logue. She's mom to Manolo, her 21-year-old son with ex-husband Joe Gonzalez.