Lie. Be dishonest. I think that's the only way. Or else be glib--write as if you actually don't take a word of what you're saying seriously--as if the story you're relaying doesn't really mean that much to you at all. I'm not saying this is the most courageous or honourable approach, I'm just saying that's what you do if you don't want to feel totally exposed. The problem is, if you want your writing to be remotely readable, you're going to have to tolerate a bit of exposure. That's the risk of putting pen to paper, and then putting your name on it (oh yes--there's a third strategy: a pseudonym).
If you are not willing to be glib or dishonest or pseudonymous in your memoir-writing, that means you have artistic integrity, which translates, ultimately, into a grateful readership. Readers can spot a glib bullshit artist a mile away, and will usually walk another mile to get away from him or her. Yes, glib BS artists will always have an audience, but it will be small and essentially adolescent. The most successful memoirists--the most successful writers in general, I'd venture to say--are the most emotionally fearless. When high-minded types are overheard in galleries or theatre lobbies making declarations about how "art involves risk," this is really all they're talking about: exposure. Not to gunfire or, even worse, the critics, but basic exposure of the most private self. When you dredge up that very terrible memory that did violence to your entire world view and ultimately changed the course of your life, your readers may not have any idea how much it cost you to put that memory down on paper and release it into the wild. But on some level they will grasp the emotional core of what you've enacted and as a result your story will stay with them for a very long time.
But is it worth it? That's what every memoirist who sits with fingers poised above the keyboard has to ask, before tapping that first key. Can I handle the exposure?
This is why some of us opt to write fiction.