Bridemaid drama HELP

So I’m supposed to get married next year.

I have my bridesmaids picked out..

BUT...

My cousin which is a year older than me, and we used to be super close growing up. She assumed she will be a bridesmaid, and that’s just not what I want..

How do I tell her I don’t want her to be a Bridemaid?

It’s not that I’m being a bitch. She just isn’t who I want in my wedding party, and she likes to make things all about her.