Why does my girlfriend hide her sexual past?

My girlfriend which I love and I know loves me is the best thing that ever happened to my life. I think she has many insecurities though and is quite dishonest at times as well. She admitted to me she has dishonesty problems overall to others but apparently not to me. When we began our relationship, she stated she was virgin however I quickly found out that was false from her past such as explicit pictures and other stories which I verified to be true. She is 20 and I am 21. A couple of days ago I assured her that if she is open to share her sexual past with me but she quickly wanted to switch the subject. She constantly hides it from me because I think she is ashamed of me however I want to know more about her and her sexual past. This is driving me nuts, what should I do?
Why does my girlfriend hide her sexual past?
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