the good news is that your story is not unsimilar to many here. There is a large selection of threads on mono/poly relationships if you do a tag search for "mono/poly."
It is completely possible to have everything you desire if you are both willing to embrace each others differences and stand back from each other enough to allow the other to be who they are. Stand back with love and admiration for each others awesomeness that is.
I don't believe anyone has to be just like their partner in order to have a relationship with them. Commitment for me comes from a love of the relationship as a separate entity rather than devotion to being what my partner is or wants. I am what I am and so are they. I love my mono partner because he is different than me and has shown himself to be committed to what we have together. I don't at all want him to be poly unless he desires it.
Your gf sounds like she is a woman with a huge heart who has the capacity to love many and express that in anyway that feels right for her. Including sexually. I wonder if her regret in her past sexual experiences is to do with feeling like her having sex with someone was not appreciated as a loving giving act. Did she feel used and that the sex she had was not taken as a serious act of love for who she was with? If so, I can relate... It wasn't until a few years ago I realised how much I had given away sexually out of love when what I gave was seen as just a sporting good time. It was an adjustment to realize and come to terms with.
Maybe this is her chance to give and receive from others in order to increase love? That doesn't mean less for you. In my experience it actually makes me love my partners more! They get the best from me in every way because I am free to be myself. Such a gift.
It is really hard to get to a place of understanding how poly works.more than ever there are places to go to find support. I'm glad you found this place.