You should take your time and not rush into marriage. You should continue to tell her about Islam and if she is sincere then she will eventually become a Muslim. You should not marry her until firstly, she does not become Muslim and secondly you do not do the Istikhara Dua or ask some pious person to do it for you; for we have no knowledge whether in the long run she will be a source of happiness for you and a good mother to your children or rather a source of tribulation, thus it is imperative you do not decide by yourself but rather ask Allah taala to guide you to the right decision by performing the Istikharah prayer.