Same here!! One thing that works for me is to slow down my speech at the start and speak with deliberate and sometimes exaggerated mouth movements, especially your lips. Keep in mind that stuttering is more common at vowel sounds than consonants because that is when all our mouth muscles need to get their timing spot on. Try to stretch your lips outward as much as possible when you pronounce the vowel sounds and that makes it slightly easy for the muscles to relax and stretch again when the next vowel hits. The key here is to keep it slow and not rush, I know it is easier said than done but you need to make a conscious effort to slow down and give yourself enough time to put together what you need to say. I noticed personally that when I try to use words that do not come up in my regular conversations, I struggle to get them out without fumbling and rolling over the words. Once you start rolling into your speech you make an extra effort to rush and catch up and that is when you end up saying incoherent things and make grammatical mistakes.