Virginity at your ages is unusual in the USA today. Perhaps you have both grown up in environments where even sexual thoughts are considered sinful. I can imagine breaking with those thoughts when entering the marriage bed might be stressful. If you aren’t able to start a conversation to address this issue, joint counseling should help. Just a thought.
Well, you said it yourself highway. Everyone has a past. When I was a whole lot younger, I had that fear of 'what if I lose interest' as the newness does wear off. BUT, what I know now is that when you have a person that you care for and choose to make a life, it's okay. Not every day is roses and butterflies in my long time marriage. But it is life. And we plough through it together. There is an eb and flow but once you are committed and make it your life situation, you see it differently. I've grown older with my husband and he with me. I know he has wrinkles now, hair has thinned, he's enjoyed my cooking, if you know what I mean. :>) But I've changed too. And you notice all of that less and less when you are growing older with someone. So, try not to worry about this.
Do you like her personality? Have you spent time together? Have you been through ups and downs to see how you guys react to stress? Do you talk about serious things like how you would raise a child, finances, goals? Those things are important. Compatability in most of it is important. And that carries you through.
Yes, tell her you don't care. And maybe stop talking about it between you. I never shared any experience I had before my husband and I noticed that if I hear things like that about someone, it conjurs an image. The image is a little unwelcome, right? So, just tell her that you are fine with it as all of her experience made her who she is and you love who she is and leave it at that. Do not tell her about your experience. It will be okay.
You say you think you might lose interest in her because she had a boyfriend before you (even though she retained her chastity). What makes this such an emotional minefield for you?
My suggestion is to decide whether you love her unconditionally or not. If you do, rejoice in her love and remember, the past is the past. She is just remorseful that her past events were not all with you because she wants you and you alone. I would suggest reassuring her, and taking it as a compliment.