First of all, I’d just like to say, Welcome to the Church!
I think the problem is that if you are not in full union with the Catholic Church, ie its dogmatic teachings, than you cannot fully participate in its sacraments. The Church teaches that it is wrong to receive communion in the state of mortal sin, that extra-marital sex is grave matter, and so, with full knowledge and consent is a mortal sin, and that to confess something without the intention to stop is also grave matter. I think the reason people started by saying you should not be entering the church without acceptance to its teachings is because of the way you suggested entering it, which is not in line with them. If you are willing to refrain from sexual activity until you and your girlfriend until you are married, and, if you are already baptized, confess the relations you have been having there is no impediment to your entering the church.
Is your problem merely a pracitcal matter of what action these circumstances require? If so, then the above should answer it. If you would like to discuss the reasoning behind the church’s teachings, just let us know and we’d love to try to help you understand!
Good luck with everything! I’ll keep you in my prayers
I just realized I did not address the issue of cohabitation. From what I know of your case, I think you would most likely be allowed to continue. As was mentioned above, talk to your priest about it and explain the situation. As I understand it, it is not cohabitation as such that is wrong, rather the extra-marital sex, near occasion of sin, and scandal that could be caused. Becuase of this, I can’t see why you wouldn’t be allowed to continue living together because of serious reasons.