Memoir writing is often practiced by people who are not otherwise writers and who will never otherwise write anything else. In workshops that I lead, I am often surprised at how demanding these writers are on themselves. There are even times when a writer will not turn in a piece of writing because it was not good enough-and that in spite of my having told the group that the writing they would submit would still be in its first draft stage.
Give yourself permission to write a rough first draft. Write pages and pages in which you describe the who, the what the where and the when of the story. Later, as you rework the piece the why will be written in.
I think of the first-draft stage of writing as “fixing” the story in the same way that in days when photographs were fixed by chemicals that stage was important if the image was not to be lost. You first draft is the stage when you “fix” your story, keep it from being lost rather than make it into a masterpiece.
Perfectionism is not a virtue at this stage. In fact, it is often not a virtue at any stage if by perfectionism you mean to fiddle with the story forever. No, better to keep writing for volume, to get the story down, to get the whole sweep of your memoir written. Quantity has this going for it: it will encourage you to keep writing as you see your pages stack up. You will have a tangible experience of your efforts adding up to something.
Quality will enter in later-as it must. You will rework your piece for various stylistic elements and eventually you will have a memoir that you are ready to launch into the world, but for now get your first draft written-and give yourself permission to let it be a rough first draft.