You don't stammer _because_ you have low confidence, fear of stammering or embarrassment. You have low confidence, fear of stammering or embarrassment _because_ you stammer. Keep addressing your stammer directly! That's the way you'll improve. You need not look for any other explanation outside of your stammer. Your stammer is what it is - and you need to keep working on it to get a grip on it. There's nothing wrong with you. You just have a stammer. Don't let it drag you down. Believe in your effort to manage it!