How do you stop speaking too fast when you're nervous?

Every piece of feedback I've got from mock interviews says the same thing: good content, but I speak too fast. When I listen back to my recordings I can hear it too: the first 30 seconds of every answer are rushed, then I slow down once I relax.

Things I'm trying:

• A deliberate one-second pause before starting any answer.
• Starting with a short first sentence ("Sure. The main reason is…") so I don't launch straight into a long one.
• Recording answers and counting the words per minute, which is honestly a bit depressing.

Has anyone actually fixed this, not just been told about it? What made the biggest difference, and did it hold up in a real interview when the nerves were real?