Dear Santiago,
Given the situation you describe, there is going to be heartbreak, no matter what you do to try to avoid it. It is will be either the wrenching heartbreak of separation the moment you decide to pursue your vocation seriously, or the slower, constant what-if heartbreak if you decide not to discern closely and go ahead in life leaving that all-important question unresolved.
Since priestly celibacy entails the absolute, total and exclusive donation of our heart, I see no other way to reasonably discern your vocation except by making the break and testing to see if you are able with God's grace to center your whole heart and your whole life on Christ alone and his work. Of course, I am supposing there is no definite, positive sign at present that you dont have a vocation because if there were I imagine you would not be asking yourself the question.
This is hard medicine, admittedly, but I think the sooner you place squarely at the center of your reckoning that a vocation to the priesthood entails the voluntary offering up of something that is very good in itself (a good relationship and eventually marriage with an excellent person) the healthier your decision is going to be either way. You may perhaps come to the conclusion that the sacrifice is too much at present. The natural tendency is to think that things will work out if we just give them some time, so no need to do anything drastic or upsetting right now. Sometimes they do, but more often than not what suffers is the possible vocation.
In your case you have an extra element on top of the normal difficulty, the fact that your girl-friend has not received the gift of the Catholic faith and so is deprived of the true source of comfort in her own distress at a separation. The priesthood is so much more than being a minister precisely because of its center, the Eucharist. She can understand your desire to serve Christ and preach his word for that is what she is familiar with, but acting In Persona Christi, the reality of the Eucharist, becoming Another Christ through and trough, the spousal relationship of the priest with the Church..., these are dimensions the reality and beauty of which may still be hidden from her. If you want to help her understand and respect your decision give her some materials to read on the Eucharist and the Priesthood, and connect her with a good priest or layperson who will be able to help her understand what she is reading.
God bless, | 
