Arnold Schwarzenegger has just released a book at the age of 77 titled something like 77 Lessons for life.
In there he has a quote that applies to getting better at open mics, I certainly found it to be true
“Reps, reps, reps. You might think you only do reps in the gym, but repetitions are the key to life. Whether you want to improve at speaking in public or reading books or just eating better, you will need to do reps. Whatever you work at, it becomes easier and less uncomfortable with every rep you do.”