I think you're doing fine, Celticfan. To me, you've demonstrated that you're convinced of the truth of Orthodoxy, the exclusion of all others. At the same time, you care for this girl and want to have a relationship. It sounds like you'd like for her to convert, but have serious doubts about that possibility. I think all of that is healthy.
Of course, the best circumstance is for Orthodox to marry Orthodox. This is most highly prized. Of course, in our times (and especially location, here in the west) that's quite difficult. We don't have a big pool of Orthodox people to pull from for romantic relationships. While it saddens me, we sometimes may have to marry Roman Catholics, or perhaps even a sacramental, confessional Protestant (such as a high-church Lutheran or Anglo-Catholic). Not ideal. Not ideal at all. C'est la vie. As long as both parties are respectful of each other's faith (and we're not out marrying JWs, Mormons, agnostic/atheists...or even Evangelicals or Presbyterians/Reformed Christians), it is possible.
Though, the concerns of raising a child in a split-faith household are very real. The Orthodox Church demands they be raised Orthodox. The Roman Catholics that they be raised Catholic. What do you do with that? Of course, the obvious answer you're going to hear from an Orthodox Christian is...raise the kid Orthodox! This is best, though might be hard. It's definitely something to talk about with your significant other when/if the relationship develops to such a level that marriage is a serious consideration. We should strive greatly to raise our children in the True Faith.
However, I think one thing should be said. Must be said. It's already been demonstrated here. DO NOT raise the kid in both faiths. It does lead to confusion (and yes, to heresy). The kid might end up in one of them, or might equate them together, or end up something else entirely (or nothing at all). Besides, raising a child to be both Orthodox and Roman Catholic is being unfaithful to both of those churches, which do not teach such equality. By encouraging such thought, both parents would be betraying their respective faiths.
Best wishes to you in this difficult circumstance. May God give you wisdom!