on the trek to find the beast simon reflects on his inability to speak at an assembly.. without that dreadful feeling of the pressure of personality. what does he mean by this? what is simon scared of?

Simon is Golding's Christ-figure meaning that he embodies the traits Christ was said to have. Simon likes all the boys including Jack. There is however always negative energy and the threat of violence. Simon can't bear this. That is why he prefers to be alone. Explaining an idea so evolved and metaphorical as mans' inherent evil is way beyond the others' comprehension. Most of them mock and jeer at what they do not understand. Simon hates this bad energy.

Also Simon is scared the boys will ultimately destroy each other.