"Michael doesn't live in LA, so it was difficult to maintain a long-distance relationship," an insider tells Us. (A rep for the actress has no comment on the split.)
Us exclusively broke news in early September that the two were an item. "They met through a friend. They've been seeing each other for a couple months now," a source told Us. "They are great together," another added. "Super sweet."
Garth, 41, and Shimbo -- a CEO of a San Francisco-based media company Planes, Trains, and Automobiles -- weren't shy when it came to their relationship. The couple often posted pictures of one other on social media, and also attended the Comedy Central Roast of James Franco together in Culver City, Calif., on Aug. 25.
The Mystery Girls actress has previously been linked to photographer Noah Abrams in August 2010 and indie-musician Jeremy Salken in January 2013. She split from husband Peter Facinelli in March 2012 after 11 years of marriage. The couple finalized their divorce in June 2013 and have three daughters together; Luca, 16, Lola, 10, and Fiona, 7. The Twilight actor has been dating Thor actress Jaimie Alexander since November 2012.
(E! Was first to report the split.)