Firstly, I would like to applause Az231 for making the right decision by ending the relationship because in the long run it's does matters.
If you stay with this guy such as being in relationship with him before marriage, in islam that is wrong. Even if the guy is a muslim or non muslim, what wrong is wrong. Allah is protecting a woman's virtues and honour by stating only marriage is acceptable not cohabitation. Because a marriage is a sacred union between a husband and wife which cannot be broken. Whereas there is no legal contract with cohabitation for instance, you are not responsible for your bf and he is not responsible for you. In a marriage, both of you are responsible for each other.
I am a strong believer in Allah and Islam, and please believe me when I say I am not judging you instead my heart goes out for you. My advice is choose wisely. This is Allah's way of testing you, yes you love this guy but do you love him more than Allah? Will you choose Allah and follow islam or will you choose your guy and lead a different path? That is your choice alone and no one in this forum can decide or choose or force you.
For me, Allah is most important being and I love Allah more than anything, more than my parents and my grandparents. Because it's Allah, who gave me my parents, my grandparents and my whole family members who I cherish more than anything on earth.
You ask az231 why he chose to split from his gf, well I don't know what are his reasons as only he can explain that. But, just because he can marry a Christian woman does not mean he should. Suppose Az231 does marry his girl one day then when they have children, which religion will they follow? Christianity or Islam? That would confuses the kids. Since their father is muslim and their mother is a strong believer of Christianity. Therefore it's not that simple. If I have kids inshallah, I would want them to be raise as decent Muslims who are devoted to Allah therefore my future husband (inshallah) had be muslim in order for him to understand and share my enthusiasm in islam so he and I can raise our kids together (inshallah this will happen one day) so they share the same strong principles as their parents. Hope you can see why it is not as straightforward as we would like things to be.
Best wishes