You're going to have to tell her all of that, especially the part about how much more happy and fulfilled you'll be when you're not being dogged by this unmet need. And the part about how your relationship with her should be expected to blossom in all kinds of new ways because you'll be so much happier and freer. Tell her all of this. Even though it will be difficult, tell her.
Make sure you tell her again and again that she's not in any way insufficient or inadequate. Make sure she really understands how precious she is to you. But tell her.
My partner of fifteen years and I are really solid and happy with one another, even as I am exploring loving intimacy outside of our dyad. He's cool with it. We're good. I even sort of have a girl friend! Well, "sort of" because we haven't been face-to-face yet, as she lives very far away. But we're really close, good friends who also share a romantic attraction. ... And about two weeks from now I'm taking a multi-day trip with an old boyfriend I haven't seen in 16-17 years! A man I'm still in love with after all of these years. (Though perhaps we'll just be good friends?)