@john88,
I presume you have always been somewhat socially awkward?
I'm not saying that as a bad thing - many people are that way. Looking for advice when you can't work it out is the right thing to do (in just about any circumstance).
Personally, I would leave the girl alone. Many people, boys & girls included, don't know how to say no - the rights or wrongs of this are not really changeable, but realising that this exists is. Many people (particularly younger ones) feel like they are a bad person if they hurt another persons feelings, and so they say one thing while meaning another, hoping the other person will just 'get the hint, and' go away.
It's not particularly fair, and it's not 'nice' (particularly to the person on the receiving end, who is told one thing but experiencing another), but it just is.
She was interested, and now she's not. Simple.