Another point to throw in there is to stop focusing on the outcome and start focusing on the process involved. Are you focusing on winning a point (outcome) or are you focusing on setting up the point in your favour, so that you can play to your strengths against his weaknesses (process).
When you start focusing on the outcome before undertaking the process, you'll start thinking too far ahead, and you'll choke. The reason you choke at 5-0 is because the outcome is closer than at 3-0 (the outcome being winning). Take away the outcome and focus on the process of each point like you do at 3-0 up