Before you speak, think about the most relaxing thing you can. Go there mentally. Make the listener wait an extra second, it’s fine.
Then, forget all about the sentence you want to say. Pretend you are only going to say the very first syllable of that sentence. Say that syllable gently/a bit softly. After you’ve completely said it, the rest of the sentence can come out gently as an afterthought. Each time you need to take a breath, re-do this.