There's a difference between a memoir and an autobiography. A memoir takes a snapshot of your life. A moment, a theme. The autobiography describes all of your life. So writing a memoir is easier because you just focus on something in your life that was really important to you. It's more manageable and you don't have to share parts of your life you don't want to share.
For example, you could write about your youth and the highlights from that time. Or you could write about your travels. Or you could write about your education, work, hobbies, etc. By focusing on that theme, you could capture quite a bit in a shorter amount of time. Also, who remembers everything that has happened in their lives? I don't. I don't remember what someone told me a few days ago, or what I ate a week ago. But I do have images of things I experienced years ago that I still remember, like my first day in kindergarten and I couldn't speak English and cried, or the time JFK got shot and I arrived home from school to see my mother crying in front of the TV as she heard the news. I remember the time my little sister fell from the slides and I had to carry her home, or when children chased us home from school. Images that are strong in your mind - those are what you write about.