Girlfriend has regrets about not sleeping with other guys before dating me

I'll try to keep this simple but apologies for the long post. I'm 24 and she's 21, this month is our three year anniversary. We have lived near each other the whole time in the same small town as I'd finished uni at this point and she chose not to go for reasons not relevant to the relationship. To this date things have always been great and despite the odd hiccup here and there we've never had any doubts. I've gotten to the point where I'm certain I want to marry this girl one day and spend my life with her. She's said the same thing about me on countless occasions and we always talk about the future together. I'm at the stage where I'm getting settled in life now and am beginning to think about getting my own house and having her move in. Currently we live at our respective parents but we see each other almost every day with the exception of when she's working late shifts (she does shift work and I work weekdays 9-5) and when we're out with our respective friends. Last weekend she went out with the girls from work and has been funny ever since. She told me after that she wanted space for a few days to work things out which I gave her although she worked most of the week. Today I popped round and had a proper chat with her. She told me that I'm the love of her life and she has no doubts at all that she wants to marry me etc but she's worried she'll get to her 30s and look back at her 20s with regret because I'm the only person she ever slept with (I only found this out a few months ago, before then she'd told me she had a few short physical relationships with 2-3 guys but that wasn't the case, she was just embarrassed about being a virgin when I've slept with 4 other people, not a huge amount I know but I'm happy with the way things are). She was with a guy for 2-3 years from the age of 15-18 but never slept with him for various reasons. She says that she's worried she'll get deeper in the relationship and never know for 100% certain that I'm the one because she's never experienced any other guys to know what to compare against. She said she's not thinking of leaving me and she would never cheat on me, she'd only ever be with someone else if she was single again, but she doesn't know what to do because she doesn't see how she could give her children advice in future about needing to go through a lot of frogs first to find your prince because she's never experienced that herself. She said she was really lucky to find her prince first time but now it's also a curse because she didn't get to enjoy her youth fully before finding me. let her hair down and try other things on the menu before deciding what she likes etc I don't know what to do. I appreciate her honesty and it explains a lot of her recent behaviour. She said she only starting thinking about it this week but now it's overwhelming her hence the sudden need for space so that she can miss me again and remember how lucky she is. But even if she gets over it this time I worry that it'll just crop up again in future time and time again. I don't want her always living with regret or resenting me for stealing her youthful experiences in a way but I can't exactly tell her I'm ok if she wants to leave me, sleep around a bit and then see if we can work it out at a later date. Life just doesn't work like that and, quite rightly, the thought of her being physical with other guys terrifies me. I'm a little bit lost and I also hate to see her struggle with it. I just want things back to normal and to continue moving on with our relationship and life together. Any thoughts, advice or similar experiences would be welcome. Thanks

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