Train yourself to use concrete details rather than abstract concepts. Take another leisurely look at what you've written. Highlight abstract phrases such as "I was afraid," or "we had a pleasant time." Replace these abstract words or phrases with something concrete, such as "my hands shook and my mouth felt stuffed with cotton" or "We drank Chardonnay on the Chatterley's sailboat, and as the sun set Rachel and I walked hand in hand along the boardwalk." Was it "a hot day," or was "the asphalt so hot that your flip flops stuck to the tar"? You might go back to the professional autobiographies you're reading and type out a few passages from the books for practice. If you're watching for it, doing so will focus your attention on the concrete. Most ordinary readers won't think about whether or not you've used abstract or concrete terms. They will know whether the writing is boring or interesting. Writing an autobiography with concrete specificity will only improve it.
Develop a central theme, main idea or thesis that clearly explains the purpose behind your autobiography. For example, your theme might be about how you overcame obstacles to become a stronger person, or it might explain why a particular passion -- such as mountain climbing or rescuing animals -- is so important to you. Back your theme with details and background information about your life experiences. For example, you might explain that your dad is in the military, so you've had to learn how to adapt quickly to new schools and unfamiliar environments. Or, you might discuss what it was like growing up in a poor, single-parent home, but how your family was always willing to take in injured or stray pets.