Here’s something that may take just a little Work. Practice laying on your back, place something flat on your stomach, and learn to breathe deeply so that your chest stays still and your stomach rises instead. Then do sitting up but slouched back, then sitting up straight. Initiate your voice on the exhale of a stomach full of air, and repeat. When you run out of air, pause, and breathe in calmly. You’d be amazed how much of a better public speaker you are when you take pauses to breathe. For more help look up diaphragmatic breathing for fluency This is not a 100% or permanent cure. Just in my experience it’s the best solution for short term increases in fluency on short notice. The more you commit to it, the more likely it is to work