You do realize that you are partaking in irrational thinking, right?
- You had sex with someone else, yet your upset she did too.
- You've had sex with other people, but you're upset that she agreed to tell you about a sexual encounter after you asked her twice to tell you.
I just wanted to highlight those, in case you were unclear what the irrational thoughts were.
I'm just teasing you some.
You feel a certain way about how she acted because it casts doubts on your value to her. You wanted her to "wait" for you, to be less experienced than you, etc. But understand that it's your ego talking, and not reality. You don't want to admit that she was off living her life just fine without you in the picture. Maybe that scares you, causes you to doubt yourself, and your abilities. Maybe it causes you to feel insecure in the value of what you have to offer to her. She's already shown she can take it or leave it.
I don't know what to tell you to get over it. You won't unless you have confidence in who you are, and your value to yourself.
And I completely don't buy into that philosophy of keepign past sexual encounters secret. I think it shows a lack of maturity, and a deep insecurity in yourself.
But, whatever.. I'm just one opinion.