You didn’t say that you and this woman ever dated, so I’m going to assume you were good friends. Because you’ve never shown her your true intentions, it makes a lot of sense, from her perspective, that she’s now got a boyfriend and can continue her friendship with you.
You needed to let her know that you wanted to date her. 😳 Since you’re just a friend, I think you have to either accept her relationship with her boyfriend and decide if you want to be a third wheel, move on, or throw a hail mary and very late in the game, and ask her out on a date.
Your timing is really off, so I don’t think she’s going to want to date you now that she’s already reunited with her ex and is planning to move in with him.
Get my book, Date Out of Your League, [url]http://www.askapril.com/relationship-dating-advice/date-out-of-your-league.html[/url] so that you will know what to do next time you meet a woman that you like and don’t want to lose her. Maybe this experience will give you the gumption to step up to the plate next time and ask a woman you like out on a date so you don’t lose her. 🙁