Exposure therapy. You need to be talking as much as possible in situations you're super uncomfortable. Meanwhile, learn how to mask. Switch headspaces from you to an authority character. You need to be performing, not taking. An way I've found to do this is by smiling and saying "hello" in an elongated way, singalong to the other person I'm speaking interestingly. On the day of the interview, you need to be talking the whole day before the interview about the topic you're talking about. You need to encourage the flow state, and be in it during the interview. Then, go into the interview as the other person, the character. I've used this technique for many interviews and when I teach. I stutter far more in my normal life than when I'm working specifically because I mask at work