Journaling is a worthy element of the writing life. The material is right at hand, all those things that have happened to you. Or your journal might run to dreams and wishes, or deconstruct the events you've been witness to, yesterday or long ago.
A journal though, no matter how carefully and faithfully kept, is just a single tool in the crafting of a memoir. Just because you have four decades of bound journals in your closet doesn't mean you're ahead of the curve on writing a memoir.
Journaling, by its very name, sets out an episodic structure of storytelling. On this day, once upon a time, these things happened, and here is how I feel about them. If there's detail in your journal entries, it can help you recreate and remember parts of the story you captured in a journal.
Your memoir is both bigger than that journal, and smaller as well. A memoir is bigger because the memoir gives us context and meaning to surround events in a life. A memoir is smaller than a journal, especially a box-load of them, because memoirs examine a slice of a life. You might have a journal that kept note of the year you kicked cocaine, or the year you started that horse farm that you eventually sold to the mall developers.
You need more to make a memoir. You're likely to research that special year of journals to search out the details that are not readily available as you write your memoir. The details might be sparing. Journal entries don't often include the smell in the air or the cast of the light in a room, or what the drug counselor wore every day there was a group meeting. Most of all, the journals don't revolve around a theme, unless you know your life's patterns and prejudices before you take down the events and feelings.
A journal makes you aware of these things, if you've chosen to be faithful to the unseemly truth. Those moments where you lost your footing, as well as the people who found the grace to help you regain your place. The details about your antagonist in the story, the character who kept you away from your dream, might include some passages about your own shortcomings. We crave those moments of vulnerability in a self-aware story. It's not natural to record those in a journal.
Journals don't usually have a narrative arc, either. My life began in this situation during that year. It fought its way through these conflicts. By the end of that year, few journals might read, all that blocked my path had taught me something. Those are reflections and summary that don't come naturally to journal writers. You may be the exception. Perhaps this advice here will let you tell your story in your journals differently.
What do you need that's not already in your journals? Sensory information. A vivid antagonist you battle against, and a vulnerable hero. A surprise that changes the memoir's outcome. Backstory for the characters in the memoir, so we have context about why they act as they do. It helps to have a sense of when to use summary, scene, and exposition. Plenty of journaling goes into detail on exposition. Scenes don't usually grace the pages of journals. The aftermath of scenes, the summary, might be in those journal pages. It can be difficult, however, to know the aftermath well just days after a scene. You write a journal day to day, sometimes without a look backward.
Memoirs are stories rich with reflection, an element which is more genuine after time has passed. If you're lucky enough to have journals about a difficult or an amazing season or year, or even several years, you have raw material. It's like reporter's notes, though, even when it's coming off the pen of a journalist. Making the story of the memoir is another, deeper step you will take.