The traditional moral prohibitions on premarital-intercourse are derived from the assumption that it will be damaging to the relationship and by making it excessively physical take it out of its proper context...the sole purpose of marriage is not sex, not that sex is wrong, it's a good and wonderful thing, but a marriage needs to be more than that.
And, seamus, this is all good and proper; however, are in a particularly difficult position from that perspective. Because of the expectations that have developed, abstaining from sexual relations may have a substantially negative effect, negating any possible benefit. In such a difficult situation one cannot rely on abstract moral norms for some type of cure-all guidance. The details of the relationship, the expectations based on past experience, and the needs and wants of BOTH partners must be taken into account. I submit that no one, including your priest, is in a positoin to give an easy black and white answer to this (like many human relationships) complex situations. But, ultimately, you and your girlfriend are probably in the best situation to work this out between yourselves; just keep in mind, the solution doesn't have to be all or nothing, black and white, it could simply be a matter of emphasis in the relationship, for example a desire to place priority on improving the personal and spiritual aspects of the relationship, but a gradual move, not a sudden change in the terms of the relationship. Trust yourself, trust your partner, I'm sure whatever decision you two make will be the right one for your context and for your relationship.