Another possibility might be a compromise on both your parts. It doesn't sound like she'll ever be interested in a triad, and it doesn't sound like you'll ever be interested in an exclusive relationship with her. How would she feel about you being poly and her being monogamous?
It might be beneficial to have a really difficult talk about the fact that you'll never be happy knowing that the relationship can only ever be exclusive and that in order for you to remain true to yourself, you need the option of one day being able to live a poly lifestyle. Then you can make the decision together whether to end the relationship or to keep poly on the table.
In other words, you don't need to feel that you are single-handedly responsible for ending the relationship because you want to be poly, and the converse is that she doesn't need to feel single-handedly responsible for ending the relationship because she won't allow you to be poly. You would be making a mutual decision that you have incompatible goals in your relationship and that it would have to end.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker