Sometimes you have the opportunity of either writing what you are going to present or thinking what to say before actually speaking.  Use the lead up time wisely.  If you are going to write your speech, brief or lesson do not insert difficult words in your verbiage.  If you have problems with the letters "m" and "b" do not try to say "Mel Gibson played a mean character in the movie Braveheart".  Choose your words carefully.  If it is an impromptu situation, and for example you have difficulties with the letter "c" you can always say "my automobile is red" or "I have a red Honda". There's no need for you to try to say "car" if you know it is one of those difficult words.  Yeah, yeah I know what you're thinking "My speech therapist told me not to use word substitution". Do you always do as your told? That's what I thought.