A source tells E! News that maintaining the relationship "started to get hard with both of their schedules," adding that they "remain on good terms."
According to IMDB, Stowell has at least three new films in post-production while Nina is currently filming "xXx: The Return of Xander Cage" with Vin Diesel, Sameul L. Jackson and Ruby Rose.
The "Vampire Diaries" and "Whiplash" stars split in late January or early February, E! reports.
On Valentine's Day, Nina shared a photo showing her with six female friends enjoying some drinks and one another's company.
"Sunday Funday," she captioned the pic. "#HappyGALentinesDay."
Nina, 27, and Austin, 30, made their red carpet debut as a couple in October at Austin's "Bridge of Spies" premiere.