Well, there are a few recipes for falling out of love.
One is to just make yourself lame. But you've got to be able to stomach that long-term, and then deal with the disrespect it elicits without behaving in a strong way in the face of it, and there's a risk behaving this way long enough affects your real-world behavior.
So this isn't very practical.
However, there is another path:
For instance, what would happen if you told her you really do care for her, but after trying this monogamy thing for a few years, it just doesn't work? And that you do not want to lie to her and hurt her and cheat on her, but sooner or later you're going to behave like a young man again?
At the same time, be less available. Take longer to respond to texts/calls. Put less emotion into them. Spend less time with her every week. Be more distracted when you do (watch movies, play games, etc. Many girls in particular do not like men playing games around them and ignoring them).
She is in love with you because the cost-benefit ratio of the relationship is a favorable one for you now.
Change the ratio, and you change her assessment of how much she really wants to stay in it.