The couple recently ended its on/off relationship after six years of dating.
A source tells UsMagazine.com, "She's moving on... because she didn't think he was The One."
Bush was previously married to fellow co-star Chad Michael Murray in 2005, while she also romanced another castmate, actor James Lafferty.
The Day After Tomorrow's Nichols broke off his engagement to artist Claire Oswalt in 2004 following a seven-year courtship.
