Are never certain. They need work. They need effort. At the beginning they seem next to impossible to do. Tough things are like that.
Same with dealing with stammering. Knowing that you will increase your confidence is not certain at the beginning. How can it be? You’re dealing with a pretty unpredictable part of your life.
You need to do the things you’re afraid of. With the experience, you’re confidence will grow. The unpredictability and doubt will slowly begin to change into certainty.