The problem with keeping in touch with her, is that the longer you are together, the more difficult it'll be to break up. And even if you don't care of the religious ban on intermarriage, that relation will mostly likely bring suffering, not happiness. However you must not bash her with Hallachic reasons, since you don't keep Torah Hallacha, and you'll sound just as an hypocrite. She may think, he cares about Judiasm for me, but he, himself is not a Noahide. Since Catholic reasons against intermarriage are different. You'd better use rational arguments about the problems it'll bring for children education, home life and so on. And even better just break up, or avoid meeting often, and when you do see each other, just talk like friends. You can also say that you are simply not sure if you really were in love with her.