I also have had difficulty with st words. I would get caught in the s and just prolong it. It was also with sc words like “school” and str words like “strawberry.” I made like 20 phone calls to different restaurants, etc. and used st/str/sc words to practice it. Now I’m doing better with it. For “strawberry”, for example, I try to quickly combine the first three letters (str), because if I hesitate and start slowly with the s, then I’ll end up prolonging the s. I just have to fully commit to pronouncing str