It seems to me, having observed other's retirements, that you still need to have a purpose, and a community of people to be with. This can mean many different things but having a compelling 'why' for what your are doing is the root of motivation. Ideally the community of people are also in a similar situation and working toward the same purpose. It often takes people a while to find such a thing.

It seems that you already know this but perhaps spelling it out is helpful.