Figure out how to be her best friend while still being able to fall in love with someone else. This might take you years, it took me years. One of the things that really helped was to realize all the ways in which we were totally incompatible. Eventually your emotions just kinda ‘detach’ from the person and you can move on.
One cool side effect of this is watching other people fall hopelessly in love with a girl you used to be in love with and moved on from. In my case it’s like, “good luck with that bud, you’re in for a rough ride.”