Your personal statement for undergraduate or graduate applications should help fill out your application by giving a sense of who you are. The admissions counselors don't want to see a list of your achievements (those should be clear from your transcript and the rest of your application), but rather a piece of writing that expresses you.
Who are you?
I still ask myself this question (most ridiculously when I'm clothes shopping; do these jeans represent my inner self?) and it isn't an easy question. You don't need to - and can't - express every aspect of yourself. But you can give a sense of yourself through a focused essay. You might choose a single experience, describe it clearly and offer some analysis that connects the experience to your mindset and the world around you. You might explain a blip on your transcript in the same way (did your grades go down one semester because of something that happened?) What about yourself - your history, goals, aspirations, experiences - is missing from the short answer questions throughout the essay?
To brainstorm ideas, you might start with your resume. Then, expand your resume by adding in other achievements that might seem smaller (and irrelevant to a formal resume), but help to show the reader who you are. Did it take particular courage to try out for the improv group? Even if you weren't selected, did you learn something from the tryouts?
For more help, you might find it helpful to work with me as a private writing coach. I can ask you questions and provide writing assignments that will help you to brainstorm, draft, and edit your essay. I look forward to hearing from you.