I want to tell my 9 year old that I love my girlfriend more than her
That's really sad, do you want her to know about this?
I just wanted to let her know that it's going to be a weird relationship from now on.
While you can love whomever you want, you really need to love your child more. They are very vulnerable and need to be secure and safe, not think they are worthless and put below someone else.
It's only a small chance that it might backfire. What would happen anyway if I do it?
You could psychologically damage your daughter for years to come. The love that you have for your daughter is a different kind of love than the love you have for your girlfriend and they really can't be compared. Don't you think you should make them both feel as loved as possible?