Due to my spiritual beliefs, my highest intention when I do things is to do them as a service to God.
If I have to do any difficult task, which could be speaking in a challenging situation, I remind myself there is nothing to worry about. After all the task I'm about to embark on is a service to God.
When I take on this attitude my worry disappears.
When you do any activity, think about your higher purpose for doing so. Think hard and you will find it.
It might be to do a task as best as you can. Or it might be to help someone else.
Is this higher purpose stronger than your fear of stammering? If so, where's the worry?