Sounds like you two are doing a lot of remote texting and calling but no in person contact. the problem with that situation is that our brain (read: imagination) makes up all kinds of missing details about the other person and the relationship is built on an illusion.
Now... sounds like you had the possibility for a relationship, and then suddenly she said 'wait, I have a boyfriend". some girls say they have to wash their hair so can't go out, when in actuality they just don't want to go out that night.
My point: if I was in your situation I'd connect with her and get with her in person ASAP for a heart to heart. see how it feels, see if there's a real connection there, and find out in person if there really is another relationship that she is actually completely in.
You can't do this stuff over text or phone... we humans are pack animals, we need that human contact. You need to see, feel, experience the other person to really get what they're saying, how they really feel.
That's just what I'd do if it was me...
-- Dan