Is it possible that you're speaking too fast for your mind to catch up with your speech? Try to be deliberate & remember to breath. A friend of mine did Pranayama & he claims it has helped him immensely. But he had to do it for about 6 months to see a difference. Finally, I suggest you to keep practicing, give a fuck to whether you win or lose now, never stop. Do not avoid such situations just because you *think* you'll lose. Go in with the conviction that you are right to react as you had to. Leave the results to nature. Just do your thing & argue / say through your stuttering.