An Apatheist is a kafir and you cannot marry her according to the Islamic law.
It's quiet vivid why she is so messed up after you stopped associating with her while you are not. Anyone who is associated to Allah (azj) and His path should and would be as strong as you stand today.
Also I find it wise to mention that she should be introduced to Islam for her tranquility and prosperity and not for you! In that case her acceptance of Islam won't have much meaning to it.
Please don't feel guilty about this. It isn't because of you; It's the denial of the Natural instinct that her creator has blessed her with and she is determined to turn her heels on it.