What can I do if my sorta boss/coworker continually speaks over me?

You have an excellent opportunity here:

You have a genuine living, breathing example of a narcissist in your midst.

These creatures are very easily manipulated. Why not use that to your advantage?

Appear to agree with her, approve of her, like her personally. She will find your “opinion” of her irresistible. She will never question a positive attitude toward her.

She’s putty in your hands.

Here’s an excellent document to help you understand your specimen:

https://en.wikipedia.org/wiki/Narcissistic_personality_disorder

My guess is that she is probably more insecure than yourself, and overcompensating in front of you. Odds are she will trip herself up. You can choose to either aid her in facilitating her upcoming embarassment, or try to unravel what it is that drives her to behave as she does. Remember, few of us deliberately choose to be disliked.

Is she deaf? I had a boss who talked over everyone because she was mostly deaf and couldn’t figure out what other people were talking about. She could make better guesses about what people were saying if she was the one who had started the conversation. So she would constantly interrupt all conversations by changing the subject and then trying to guess what the responses were.

I’m actually surprised by the number of upvotes my comment has received.