Like you, I have attempted to write an autobiography, but have encountered similar problems.
The basic difficulty is that you know too much about the story you want to tell. If you write about some made up fiction, you usually only have some kind of core of an idea, and this core will serve as an orientation when you flesh out the novel or screenplay. But when you begin with the full story, as in an autobiography, you have to go the opposite way and distill the core of your story from all the overwhelming amount of detail that you have.
There certainly isn't one right way to go about that, but here are some thoughts I have on how to attempt this, taken from my own experience:
Try to write the logline for a movie about your life. What would that movie be about, if you had only one or two sentences to summarize it?
Try to define the theme or premise of your life. What goals have you been struggling to achieve? What is your most fundamental characteristic, either strength or weakness? What is the meaning of your life to you? Who are you, in a few words?
Do some research on the three technical terms from filmmaking I have used (logline, premise, theme), what they are, what their purpose is, and how they may be found. I have found this perspective very helpful when dealing with a too complex story idea.
When you have thus reduced your life to the bare minimum,
Write a brief chronological outline of your life as it relates to the theme, premise, and logline you have defined. Leave out everything that is unrelated to this, no matter how important it might feel to you otherwise. You are not writing a complete account of your life (which would be boring to read for someone who is not you), but a story of your life. Think about that difference every time you get lost: story versus account.
Finally use the outline, premise, theme, and logline to develop your characters and write your novel in the same way that any other novel is written. If you put yourself into the writing-a-story frame of mind, you will find that no more difficult than writing anything else.