I think many of us can relate to this problem. Sometimes when I start speaking it’s relaxed without many stutters, but with long talking it can start to get worse and then it’s a spiral that only gets worse. Mostly because I start to overthink what other people will think about it… What does help for me is to be honest to everybody and just speak out in the middle of the conversation/presentation that it’s hard with my stutters and I need to breath a few extra times to relax. Or maybe even make a joke about it in the middel of it all. It can clear the air and you’ll notice understanding on peoples faces. Most times such mini break helps, but if it’s really bad maybe you can just say to have a 5 min break. Try to make it a confident brake moment and don’t feel ashamed. Be honest and (99% of all) people will be understand it completely. Get something to drink, do some breathing exercises and continue.