I think you need to tell her, when you've been together for a while, you see a future, and you know you trust and love each other. Having been in this situation, I really appreciated my SO's honesty about this issue, and because we are committed to each other, we were able to overcome the negative feelings about it. It hurts a lot, and honestly it can be very hard to forgive someone who has had the issue in the past, but that's part of the Atonement: she will eventually, if she is the one for you, forgive you and your relationship will grow stronger because of that trial. I do also agree that there will always be the potential for relapse into that habit, so she has even more right to know, in case it does.
