Everyone says that I need to work on me; what does that mean? I'm fit, employed and well-liked. I was a good spouse and parent.
It means that you are experiencing a life-changing crisis and since you life is going to be different now--whether your MLCer returns someday or not--you are going to need to make some personal changes to adjust. You are going to need to fortify your strength, because dealing with a midlife crisis spouse will bring you to your knees even if you are a strong person; this is going to be different than normal everyday life. Many people who experienced a spouse's abandonment feel like failures--regardless of their confidence levels prior to their spouse's midlife crisis. To become a survivor and believe in your worth through this experience, you will need to maintain your Self--either to gain or maintain.
Your spouse is going through midlife crisis and will change; some come through the tunnel and return basically to their former Self--they have rediscovered who they are. Others come through the tunnel completely different; they learned about who they are and made changes which they liked and thus chose to keep. It is not your job in this to witness their changes and adjust in accommodation; remember what followed Accommodation? Midlife!
While your spouse is finding himself do the same. I know, you already know your Self. Who we are is constantly changing; take this opportunity to explore new thoughts, ideas and activities. Read a new book, take a hike, jump from an airplane, take a class. What have you put-off doing because your responsibilities as half of a couple got in the way or because your spouse wasn't interested or simply because they made it impossible? I had put off painting the interior of the house because Sweetheart has trouble handling clutter or when things are out of place--furniture being piled in other rooms or covered with paint cloths and unusable. (He even has trouble with mini transitions!) So I painted while he was gone and I got to choose the colors! Painting led to other redecorating and it became a personal project wherein I got to infuse my Self into the creation. What can you do?