I used to have extreme stuttering. I still stutter but i have managed to reduce it by a big margin. Before trying any of the methods/ways I've mentioned below, you need to understand one thing which is that you need to LOVE YOUR VOICE first. And you need to understand that your goal is to convey your message properly and not to speak perfectly. Watch some of elon musk's podcasts and notice how much he stutters. So first of all stuttering does not look as bad to others as it does to you in your mind. And knowing this reduces stress and anxiety. You can check for yourself- just record yourself speaking about a particular topic. For 1 minute. (Video recording). The second thing that i learned was stutter is mainly due to lack of airflow. When you stutter. Try talking to yourself in the mirror and ensure that you don't break the airflow(i.e.don't pause exhaling while you speak). And another cause of stutter and blockage is the anticipation. Your brain knows where you're gonna stutter. So when you're about to say it, your brain fucks up your speech. I have a workaround for this. Just stretch the syllable that you know is going to make you stutter. It's not the perfect solution but it works most of the time for me. I'd rather speak slower and clearer than stutter. So for example- Umbrella --> Ummmmm-brella. As for social anxiety and public speaking and presentation purpose, just speak slower and do this stretching thing and move your hands and head while you speak. Try not to stand completely still as it increases anxiety. And try not to lock your lower jaw while you speak. And last advice is- you might have noticed that when someone is confident while they are speaking, they have a different body language as compared to when they're not. The reverse also works. So make sure you look confident in your body language.