Girlfriend not interested in Krishna consciousness.
That the chanting has decreased your material desires is a good sign. That's what's supposed to happen. Keep up the good work. It's not surprising that she's not interested in devotional life, if she's not practicing it. If you feel the relationship isn't helping your own spiritual progress, perhaps it's Krishna's mercy that you aren't attached to her. You can always pray for her ultimate benefit.
Perhaps she could develop relationships with some of the female devotees in your area. The more friends we have who are devotees, the better.
To maintain the relationship by pretending not to practice Krishna consciousness (some devotees have tried this) wouldn't be helpful, since strong relationships are based on honesty. In the long run, if you plan to get married someday, you may wish to consider marrying someone who's at least open to becoming enthusiastic about devotional service. You might point out to your girlfriend that you doubt your relationship could succeed as a marriage if Krishna consciousness—which is an important part of your life now—has very little meaning for her.
I was once engaged to someone but could see that the relationship wasn't helping my spiritual life. At the time, I was so attached that it was difficult to drop it. But I prayed to Krishna that, if He thought this relationship was the best thing for my spiritual life, that He arrange for it to continue, or if He thought another girl would be better, then let that happen, or if He thought the best thing was for me to go back to being a brahmachari (celibate student), which incidentally was very nice, that that might happen. As it turned out, the girl broke up with me within a month, and I chose to be a brahmacari again.
One idea is, if you're attached to maintaining the relationship and she's attached to her idea of you but unwilling to accept your commitment to Krishna consciousness, is for both of you visit Mayapur, the birthplace of Chaitanya Mahaprabhu. You could pray to Chaitanya Mahaprabhu that she becomes a devotee. If she even goes to Mayapur once, her life will be perfect. Such is the power of the holy dhama. Then, if you end up splitting, you can be satisfied you gave her the best benediction. If you prefer, you can take her to Vrindavan.
Meanwhile, keep chanting with as much attention as you can and you'll get the full benefit of practicing Krishna consciousness.