Is your girlfriend really "lying" to you?
In the technical sense, she is not telling the truth, therefore lying. But, in reality, she is confused about her past. This is very different from doing things behind your back and lying about them, don't you think?
I mean, she was raped, she drank (or whatever) to the point of passing out only to be molested sexually by a supposed friend, she had some kind of relationship with another man that might have been sexual...she seems very confused herself. She may also be afraid of your reaction to these rather unusual experiences. These are not very comfortable things to talk about.
Are you sure you can handle the truth?
You may think that honesty is always the best policy in relationships. On the other hand, to paraphrase Jack Nicholson, "Are you sure you can handle the truth?"
My wife, who is older than I, and grew up during the sexual revolution of the 70s, has had experiences that I frankly, don't need to hear about. I "can't handle the truth." I don't want to know about everyone in her past. I don't need to hear it, and it would hurt me to hear it.
Could it be that your girlfriend was concerned about your reaction? Aren't you reacting negatively now? Why does changing her story about her past threaten you? In a court of law, when a litigator establishes that a lie has occurred he or she often uses that lie to discount everything that the witness has ever said. But, this is not a court of law.
Is it really true that her revising her story is evidence of a real pattern of lying? I doubt it. You seem concerned about trust. Can you trust her? I wonder if she can trust you with her uncomfortable feelings. If she has to revise her thoughts because of faulty memory or traumatic incidents, she needs you to be understanding and tolerant, not judgmental.
Before I call my girlfriend a "liar," I'd want to make really sure about what was going on with her feelings, fears, and memory.