You come from a diverse sexual background. Your girlfriend does not. She is afraid that you will find her as she is boring, so she is willing to go outside of her comfort zone to make you happy? Your question in a nutshell I think.
Consider things from her perspective for a minute. She is worried that you find her boring. She knows the diversity and experimentation that you have enjoyed prior to her, and in order for you NOT to find her boring, she is willing to experiment with a threesome.
She Doesn't Really Want It To Happen
I honestly think that she is not really looking for a YES answer from you. I believe she's looking for approval and reassurance that SHE is enough for you.
She probably would go through with it, if you agreed to it. But simply by agreeing, you would probably be compounding the belief that she already fights with that she is NOT enough to satisfy you.
Sexuality, I believe, is somewhat of a learned trait. We are taught from children whether to be open to it or shy from it. We learn from our own experiences whether it feels good, degrading, exciting or uncomfortable.
You Have Different Sexual Backgrounds
Every person has a different level of comfort where sex is concerned, and a level of experimentation that they feel is safe. It sounds that you and your girlfriend come from very different backgrounds. You have opened her eyes to a world that she didn't understand, but I'm going to guess that her
beliefs are still strongly within her.
You Could Damage The Trust You've Built
She may be enjoying all that you have taught her and shown her, but she might be afraid of it also. If you push her before she's truly comfortable with something, it could set you both back in your relationship and damage the trust she has given you so far.
Decide If She Can Satisfy You Alone
You also should be honest with yourself and ask if your girlfriend, as she is, without the threesome, IS enough for you.
If you know what your own limitations are, then you can help to make her feel more secure with you. If you don't believe that she is enough to satisfy you, then she will sense that and react to it.
A threesome is not just sexual, it's emotional. You said that you have shown her a world she didn't really know. In order for her to be open to you in that way, she has placed her trust in you. You should be very, very gentle with that trust. A woman who has sexual inhibitions does not open up easily in that area. When she does, if that trust is betrayed, it can be very difficult to go backwards.
She'll Feel Awkward
You are in essence asking your girlfriend to interact with you while you make love to another woman. She has no desire to join the interplay with the woman herself, so she will probably feel somewhat awkward and unsure.
If she already believes that she's not enough for you, watching you have sex with another woman could very easily compound that fear. If you climax, if you ENJOY the other woman - your girlfriend could see any of this as proof that she does not excite you enough.
She'll Regret It
You seem to know from the way that you phrased your question that she's only doing this for you. If that's the case, then she will probably regret it later. You could very easily lose her over it.
Rather than look towards something like this as a way to bring you two closer, find ways to let her know that SHE is enough for you. Telling her that you are happy with just her is a great start.