HELP!!!! I dont want to lose my girlfriend. what should i do?

Me and my girlfriend have been together for 4 years now. There have always been ups and downs in the relationship but we seem to always pull through. We are now just copping with a long-distance relationship (I moved to UC Davis to finish my last year for my BA and she stayed in our hometown.) We just finished having a pretty big argument over trust. Someone has been going behind my back and causing quite a situation amongst me and my family. Unfortunately, all the evidence points to my girlfriend (she and my mom have not had the best relationship, in which case there have been times where I had to choose between the two. Actually, the problem is our relationship and the disapproving ways my family has for her for the past 4 years.) I left out all the possibilities of having it be anyone else and immediately blamed her. We yelled and argued to the brink of her crying and me yelling. After a very painful argument, I realized it may not be her at all (which leads me to a completely different problem, but I can get that later.) I guess I tried to apologize but it was too late. She decided it was best for us to “take a break.” She wouldn’t even accept an “I love you.” I have been having problems with the family and it seemed that it could be traced back to certain letters, emails, and other media forms that have caused my family grief. When I realized that there was a possibility that my girlfriend could hold a part in this, I snapped. How could the person I defended all those arguments end up being the catalyst. It was as if all those wasted arguments with my family were thrown back in my face. But now, I see my error and that it wasn’t her. I really hurt her. Trust is one thing I know a relationship needs in order to survive, and in that moment, I lost that for her. She doesn’t wanna talk to me as of that night. I realize what a huge mistake I’ve made. She openly admitted that she would bend over backwards if it meant my family would accept her. Now I fear that I may have pushed her to the brink of our relationship. It seems I always do this. I always had a fear of losing her from the very beginning. Something about her was different, which is why we are still together now…for now. I love her with all my heart, and I could not bear losing her. She literally completes me. I made a horrible mistake. She even said it will be very difficult for her to forgive me, if she even could. How could I ever make it up to her? How could I show her I love her, and that I am truly, deeply sorry for what I did? I just hope it’s not too late….