Thank you for writing. As you quite correctly pointed out, the problem lies not with your girlfriend but with you. At 27, you’re unlikely to find someone with as many strengths who hasn’t been with a number of partners (just as you have been). The twenties tend to be a time of finding out who you are and who you want to be with. Often that sorting means trying on different relationships, making mistakes, being hurt, and sometimes being the one who does the hurting.
Instead of maligning your own character (insecure, immature, selfish), it would be more helpful for you to look at what is keeping you from taking the next step in a relationship that is so promising. I suspect that you are unconsciously using the number of her past sexual partners as a reason to slow yourself down or to not commmit. If it were not this reason, it might well be something else. If you don’t figure out your side of this issue, you’re right to be concerned that it will continue to undermine your otherwise perfect relationship.
I can think of a couple of possibilities that may or may not be relevant since I don’t know enough about you.
Could it be that you are trying to level out the relationship is some way? Your girlfriend sounds exemplary. If you don’t feel that you are as mature and well-developed as she is, perhaps your concern about her past is a way for you to feel equal.
Could it be that you have a need to have the upper hand or to feel superior? If that is the case, it’s trouble. As you undoubtedly know, a partnership is most likely to survive and thrive when it’s based on equality.
I hope you will work on this – both for your sake and hers. You wrote a very honest and open letter and this relationship sounds so promising. If you can’t figure this all out with your girlfriend, you might find it useful to have a few sessions with a couples counselor. A counselor could help you get out of your own way. Take your email and this response with you to the first session to jumpstart the work.
I wish you well.