Friend dating crush
If he's in any way fair to you, then you won't have to actually choose between your GF and your best friend. Unless there are more details that you haven't shared yet, the worst thing you can do right now is to threaten your relationship.
I spoke to my friend about how my relationship with his former crush was evolving into something bigger, and he assured me that he doesn't mind, doesn't like her anymore, and is not really in the position to say anything.
My friend will probably be trapped by the guilt and sadness, which will negatively affect our friendship.
Both of them are important to me, and choosing one over another will not make any of us happy.
Being reminded of her via you getting into a relationship with her probably didn't help, but that's why I suggested trying to avoid the topic past a certain point.
You found yourself in an impossible situation with conflicting goals, congratulations.