What that says to me is you haven't found someone new, and so you just lament the inevitable. I'm not claiming to be any better at the game, just saying the ideal would be to cheer her on while having someone new around. Girls grow up. We can't change it. All we can do is hope we contributed something for their future success in life during the precious time we shared with them.
Or actually, that "should" be one of our goals, and our longest lasting hope, a hope that lives on well beyond the time we spent together. Girl Love in future years will make that one of its priorities. The parasitic label foisted upon us by the chosen narrative of greater society cannot stand against such a selfless desire, provided it is honest.
In my life, that hope for girls I've known has always been real. Most of them turned into amazing women over time, but from my vantage point, it is because they were already amazing girls, whether they knew it or not.
Sometimes I think we are just enlisted into their lives temporarily for needed guidance or self-discovery, in return for all or some of the mutual affections we so deeply crave from them, during those years that we find ourselves so willingly at their beck and call.
Try not to lament the temporariness of it all. The inevitable heartache is the trade off for the vibrant experiences when you're still in the thick of it. The difficult endings make you a stronger and more wise person later in life, and that heartache is lessened once you can guess how your future experiences with girls will end.