That is the million dollar question. Adding an interfaith element means you have many more adjustments to make. No one knows your situation the way you do. Dress nicely, as the girl will appreciate the effort put into looking good for her, and encourage her to do the same.
It really helped to make me feel better. We met at home just before he went back to school and even though we only had a week together, it was whirlwind and we already spoke of marriage and kids. As for me, I recall spending a lot of Sundays at sporting events with my dad. You aren't engaged or married or anything, you can't just know that she won't see the light and remain crazy Mormon forever. Can someone put some sense in me. I didn't hear from him all day, is that normal. Its always for his prior commitment that we always end up cancelling our plan of being together.
She is considered "an old maid" by Mormon standards, so she may be willing to marry you--hoping you will convert someday --but she will constantly be reminded that your marriage is inferior to the "Eternal Families" of sealed Mormons, and she will fear dying and never seeing her loved ones again. You can and should share with her why you don't and never will believe in the Mormon church, and let her decide for herself, but be prepared to have that blow up in your face. If you decide to stay. Joanna has written a good answer here. If you are willing to understand and accept these, whether or not you are a Mormon, you can have a fun time dating Mormon girls. You do not want hear in jeans if you are planning to eat at a fancy restaurant. Most of us were Mormons and one point, many of us were even TBMs. That's probably true, and that would be a great thing for her, but they wouldn't see it that way.
I am really in love with him. She will not marry you until you convert to mormonism whole cloth. She might not even be living according to the moral standards of the Church.