How do I write a personal sketch in a diary form without exposing myself?
Writing a personal sketch that does not expose part of you and your life that you would like to keep private will simply entail choosing what to discuss and what to leave out from your sketch.
If you want to keep certain feelings, sensitivities, interests or areas private, shift your focus in different directions. Every person is sufficiently complex to justify pages and pages of details in a personal sketch. To avoid exposing the private parts of yourself, just choose to leave out the material you are uncomfortable with and include the material you are comfortable sharing.
Instead of talking about your love life, talk about your family. Instead of talking about issues relating to school, talk about where you like to go on vacation, what you like to do on the weekend, or your future aspirations.