Give your reader a feeling that you grew or changed because of the memoir. This doesn't mean that you should write the end in a sappy way. It just means that you can choose to write the end in an optimistic manner.
Write the end of a traumatic experience in your memoir by telling your reader how you emerged from the situation. If you write about an accident, let the reader know how you were changed by the experience, or how you survived and healed after your trip to the hospital.
Begin and end the memoir with a theme that runs through the entire story. If your story begins with a body of water, and has water as a theme throughout, you should end your memoir with the same recurring theme.
Write the end of your memoir before you begin the entire project. After spending so much time working on this one project, it can become easy to rush through the end. This can be avoided if you have a rough draft of the end ready when you get to it.
End your memoir with honesty and clarity. You may want to leave the readers with optimism and lessons learned, but you also want to be brutally honest and show all of your flaws. If you didn't follow up with your father after a reunion, or you never called the guy whose number you got when you felt brave, you should let the reader know the truth. Tell the truth and the reader can decide how they feel about it.