Just tell them you know exactly what you want to say but your brain has misplaced how to say it. Having struggled with them myself, C's and M's can be especially difficult because pronouncing them starts with either your lips or your throat closed, so they're cahn-ducive (conducive) to blocking. Sometimes I'll misspell a word in my head to emphasize the vowel, while the consonant only modified how I open my mouth to speak- a literal definition, but perhaps it makes sense for us who've spent entire seconds saying c-c-c. When you find something that works for you, don't depend on it to speak but instead try to reinforce the idea that "I can say this word reliably," until it replaces any tendency you have to avoid difficult words.