First off, just wanna say I’ve been there a million times. I know those moments are tough and weigh on you but try not to let it get you down. It wasn’t the first and it won’t be the last, but with a little work it’ll happen a lot less. Idk what type of therapy you’ve done, if any, or general tools you have at your disposal, but maybe a good place to start is focusing on the mechanics of how you’re producing that S sound. Are slamming your teeth together? Does your jaw feel tight? Maybe somewhere to start is practicing producing that S sound with your teeth/jaw apart; it greatly relieves the tension in your mouth. After working on just holding that sound, practice different words that follow the S with both consonants and vowels to feel what it’s like to move passed that initial S sound. Hope this helps. Mechanics aren’t everything, but if you can try and understand what’s going on at the most foundational level of your speech you’ll be able to start finding a better path to controlling your speech. And also, whether or not you felt embarrassed, you taught that person a lesson in patience and perseverance.