I want to cut contact with my female best friend over a new romantic relationship she can't come to terms with
How long have you known your best friend?
I've known her since we were kids. I thought we were soulmates.
If you feel that closely to her, you should talk to her about your feelings instead of cutting off ties with her. You don't want to lose someone so important to you without trying to fix things first.
She thinks she has a right to dictate who I see and talk to because of our friendship. She is controlling me.
If you feel she is being overly controlling and the friendship is no longer healthy, you have every right to end it. I would suggest talking to her first and explaining things to her to see if you can save the friendship. If that doesn't work though, it may be best to end things with her even though you have been friends a long time.