When you eventually do become 'a couple', and you are both with each other, without anyone between, it will be like any other situation. Your girlfriend will come first to you, and you will be priority for her.
It's not unlike if you had a male friend, and you had been used to three evenings of darts, and golf on Sunday. You would have to back off on the friendship, and develop interests and activities with your girlfriend.
If your ex was a guy, this wouldn't be a problem. You're not giving up on the friendship, you are putting the friendship in proportional importance to your relationship with your girlfriend. I'd bet my bippy she woulnd't have a problem with it at all.
However, maintaining a friendship with a woman who loves you, regardless that you see her only as a friend, will also be a thorn in your girlfriends side. If the ex eventually finds her own man, that might work into some sort of couples thing. But, as long as she remains in love with you, it isn't a friendship. It is very unbalanced.
I think you said your frienship has gone on for 9 years? It is time for both of you to realize that your needs and wants are not going to be satisfied with each other. Break the friendship would be my advice.
Regardless of who you end up with as a girlfriend, you need to get rid of that long standing relationship.