What should I do?

I'm 17 and in high school, about to graduate. I'm dating the most beautiful and most amazing girl I've ever met and most likely will never meet anyone like her. I've been in love with her for 3 years and I really want to date her in college but she can't date me if I'm not Christian and right now I don't know what I am. So there are a few questions I have, and I'm in so much pain thinking about these that I need someone's help. We both know that if I'm not Christian then we're gonna break up, and I think that's what's gonna happen. So should we plan a last date and try to have fun? Or should I just leave a letter on her night stand after a date and tell her that we can't be together and she knows that? (Which now that I'm thinking about seems like an idiotic way to do) but please help me, I've cried myself to sleep countless nights thinking about this, and I don't know what to do. And please don't say "talk to her about it"