I'd just like to add that you could make the choice to stay with her and try to "get over" (or more likely, bury) your poly tendencies, as she wishes. You might get a lot out of a relationship with her. I did this. I hadn't heard of polyamory, unfortunately, and only knew that I had a problem with cheating and temptation. I married my husband thinking I'd finally found a man I loved enough I wouldn't want anyone else. It took about 10 years (and the distraction of two babies) before my poly feelings came up again, and now we both struggle with the pain of it. We have to live a compromise, which we keep having to fine tune.
I don't regret marrying my husband at all, but this girl who loves you should come to understand that parting ways now is probably the least painful, least complicated path for you both. You know yourself well enough to know that this side of you won't go away. If she really holds to feeling like you are the love of her life (and I don't find this preposterous) she needs to know that this is a part of you that she would have to learn to love too, if you were together.
Married to a monogamous man 15 yrs, mother of 2, dating C 3 yrs, and in a romantic friendship with L more than 20 yrs