A good memoir isn't a life story; it's a peek into a time in your life when you had genuine feeling, a genuine experience.  Try to keep your memoir to a narrow focus on one time period or aspect in your life, ultimately drilling home a larger message. If written well, this one topic or time you went through will become universal and all audiences will be able to relate. Start thinking of writable material.  What's something you can't deny? What or who did you leave behind? What's something you did that you no longer understand? What are you sorry you never did? What physical characteristic are you proud to pass on? When did you unexpectedly feel compassion? What do you have too much of? When did you know you were in trouble? They will bring to mind the experiences you could write about.  If possible, go to the scene and relive the events in your mind. Just because you can't remember it off-hand doesn't mean you shouldn't write about it. Memoirs are all about self-exploration and there's more to you than just you, after all. You are the places you go, the people you love, and the things you have, too. This is one moment when your mind should be playing second fiddle to your heart. And if the emotions are scary, nonsensical, painful, or downright terrifying, all the better. Bringing these to the surface will help you stay in the moment and write with passion, purpose, and clarity.  If a train of thought gets close to a nerve, don't close the doors and draw the curtains. If you stop, your writing will go flat and you'll end up dancing around subjects. Take your mind to a place it may not want to go. Hiding behind those first thoughts may be something worth knowing, worth writing about. Listen to music that can metaphorically take you back in time or noticeably changes your mood. Anything that stirs your emotions and allows your mind to be absorbed back into that moment can shed light on the past. This not only gives you one or two hours a week to get mindfully organized, but it allows your writing to be organic and creative and not the therapy itself. A memoir is not to find closure, it's to be shared with others, to expose a bit of yourself. It's totally normal to feel like you're going crazy. Digging around your old emotions will surely bring them to life and make them feel real. All you have to do, then, is write them down on paper and soak in the catharsis. You may even find that the story is writing itself and the conclusion you never even saw coming is looming right in front of you.

Summary:
Start narrowing it down. Pull out old pictures, diaries, and objects of nostalgia. Allow your emotions to flow. Give therapy a shot.