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.
Stick to informal language, unless it’s absolutely necessary to use formal terms. Instead of “utilize” you can just put “use”. Focus on simple language, with one or two syllable words. The only time you should be using higher level language is if you are using scientific terms or describing a technical process. Even then, you should be writing for the average reader.
Organize what you've written according to cause and effect and action and reaction. Ordering events according to cause and effect is a natural process. Your car broke down, so you called a tow truck. That's cause and effect. You received a promotion, so you bought a house. Action/reaction happens paragraph by paragraph. He said "No," so you said "Why?" That's action/reaction. She set a wet glass on the table, so you placed a coaster under it, and remembered how your mother used to crochet pretty little coasters and starch them. That's action/reaction.
Create an outline or a graphic organizer of the major topics and ideas you want to cover. For example, you might make a time-order chart -- a chart with event blocks to organize events chronologically. Or, you might draw a cluster web -- a diagram with connected circles -- to divide your autobiography into sections, such as one for relevant background information, one for life-changing events, one for special memories and one for goals and dreams. Focus on reasons why specific events, situations and circumstances were important to you and how they made you feel.