I think she just is a private person.
She doesn't want you intruding on her private stuff.
She kept you out because it's none of your business.
Now she can't understand why you are getting in a big huff over this.
I don't either.
Let it go. You are making this all about you and your feelings.
She expected you to be pleased for her.
But she got a bad reaction from you.
"I just wish I could've shared the excitement with her when this was all starting out"
The thing is, our teens are not our 'friends'.
We don't 'grow' children in order to provide ourselves with friends.
It's NECESSARY for a teenager to be separate from their mother.
It's how they learn to eventually grow up and live independent lives.
You are the parent. She is the child.
A parent's love for a child is not the same as the child's love for a parent.
One is an apple the other is an orange.
Do not expect her to show you the same love as you feel for her.
Because it's different.
Did you burst into the room and tell your mother excitedly that you fancied someone and were going out with them?
When I hear a mother who describes her teenage daughter is her 'best friend' I already know there is an emotional burden on that daughter.
Maybe when that daughter is 35 they can be more like 'best friends' but mother and teenager - no, there has to be boundaries.