For psychological reasons (as opposed to a physical reason, as per the Flaw: Impediment), you speak with some sort of frustrating imperfection: a lisp, a stutter, a too-broad accent, a whisper, and so forth. Clearly, this interferes with your attempts to wax eloquent; System-wise, add +2 to the difficulty of rolls that involve speaking clearly. This Flaw should be roleplayed out whenever possible, and it carries over if your character changes into some other form… after all, it’s your mind, not your body, that’s causing the problem; if it is your body, then take the Flaw: Impediment instead.