Ride a familiar road that includes a stretch that can test your confidence and abilities, over and over again. Start riding that section at whatever pace feels comfortable to you. Each successive time through it, pick it up just a little bit until you're going through it at the speed you finally want to achieve. The familiarity of the road will help you build confidence with each pass, since the road is known to you and you won't be continually wondering what's coming up and if you're already way too fast for it.