the problem.
It used to be so fun to do this, to do that. Now, it's not the same. Everything is different now and I don't like it. I miss things, people, places: I miss the past. I miss the way things were.
the Zen understanding.
In the Zen Buddhist understanding, there is no distinct past or future. Thus, to long for the past as something isolated in amber is to long for something non-existent; you never had that in the first place and you never will. You are in the present moment...
...and the present moment is not one thing or one moment, as no thing is one thing or one moment. The present moment is present moments, ripples of infinite choices that spread so far and so deep that you cannot help but point to the present moment.
A Zen Buddhist thinks of time like the rain cycle: where the clouds are the past, the falling rain is our choices, and the ripples upon the ocean are the future, but where all is the present moment in a cycle.
In this way, a Zen Buddhist understands that nothing is truly gone, just the way that nothing truly is. So there is nothing to miss or lament about joys you experienced. They persist in all other things.
the Zen application
A Zen Buddhist refrains from comparing the "present" to the "past" because these are delusional conceptions of reality that lack the Buddha-like understanding of co-being.
Instead, a Zen Buddhist meditates when they are distraught, reads sutras, studies reality, and takes refuge in the Sangha (the community of Buddhist practitioners -- one of the Three Jewels discussed in the Three Jewels chapter of this book). This helps to set a Zen Buddhist straight, when they become too involved in common delusions.
It is hard to divorce oneself from the past: to quit being nostalgic and sad. Understanding reality and having confidence in the present is a powerful tool to eliminate melancholia and existential dread, however.
All it takes is patience and a willingness to learn.