Trying to read my girlfriend?

[Sophie]

1) if you do not trust her, then you cannot build a relationship with her.
proof: your doubts and the constant fights you have with her based on the past.

2) you have doubts because you haven't fully let go of the past and it is basically haunting you. I am guessing that you do not have any closure on that topic and I even bet you think she has not stopped acting that way at all. Which you could also see when she stormed out and went to a party.

My suggestion? If you cannot stop looking over your shoulder, then let her go- not for her interest but for yours. If you have to constantly ask her if she has cheated on you, then you believe that she already has or is capable of doing it at any time. Living like that is not living at all and that fear is certainly not part of the relationship.

Hope I helped.