I think i'm horrible at speaking too.
But here's how i survived my speaking SAC earlier in Term 2:
1. Memorise your lines, but allow room for minor adjustments. Memorise extra back-up lines like: Sorry, may i start again? or Can you repeat that question? Once you memorise them, you realise that it's just rolling off your tongue.
2. After memorisation comes tone. Put expression into your voice as you speak, so you convince your teacher and examiner that you are confident and passionate. Everyone hates monotone, and monotone would be easier for your examiner to detect if you memorised it or if you know your stuff.
3. SMILE! It softens up the examiners so they smile back, and you feel less tense. Use facial expressions to mimic the expression in your voice. Seriously, i think it was this that saved my butt in the SAC. My teacher puts HUUGE emphasis in feeling what you're saying.