Make yourself unavailable. Only respond to half her texts. But when you do respond, seem excited.
She will wonder why you aren't responding to every single text, even though she knows you like her. This will ignite interest in her, and she will be the one chasing you, not you -> her.
Seriously, take my advice. I would have saved myself years of trouble if I had just done that. You may not want to be unavailable because you're addicted to talking to her, well that's fine. After this girl stops showing interest in you, come back to this thread, read my post, and use my advice in the future...
Haven't you ever been so interested in a girl, but you just couldn't figure her out because she was a mystery and you wanted to know more about what she was like and why she acted the way she did? And once you got to know her, everything was predictable, and you just didn't have anymore interest in her.... You gotta change. Be a mystery, and she will always have interest in you. Right now you're being predictable so her interest is fading like a falling roller coaster.