Really hard to explain. I can form a sentence in my head, but by the time I start saying it, I already know which word(s) which will refuse to play ball. I usually spend the time to reach that part in the sentence to find another word to replace it with or a way to restructure the sentence completely. It's exhausting really, lol. Mind you that there is a difference between having problems pronouncing certain words and blocking completely. Also something that is really hard to explain. Try to imagine walking up a set of stairs. Stumbling on a step happens to everyone now and then, but a block is more like reaching step number 4 and your legs refusing to walk up to step 5. You try and try to lift your leg, but it's just not happening.