My opinion on your question will probably differ somewhat from what people expect to hear at a psychologist's office, but I'd like to share it with you.
What you called "the empty bucket inside you" is called "the will to life" by Schopenhauer, and "the will to power" by Nietzsche. Schopenhauer, in his time, demonstrated how the presence of this very will to live leads to suffering and proposed a solution in the form of suppressing it. Nietzsche, on the contrary, argued that this will is the basis of movement, and whoever has more of it is stronger.
I don't like either of these hints. The thing is, as far as I know, humanity still doesn't know what fills this "bucket" or the exact nature of this phenomenon. If humanity knew the answer to this question, the world would long ago have been filled with artificial creations—like robots with their own wills.
Christianity also has much to say about working with the will. But it's more about harnessing your willpower than about creating it from nothingness.
In the army, there's also a way to awaken your willpower if it's completely absent: create such unbearable living conditions that it's born, even on the brink of losing your life.
I think in your case, it's all a bit different.
From the way your post is written, from the way you described it, I believe you've done a great deal of self-reflection. And you did it with the goal of filling that "empty bucket," or more precisely, finding something with which to fill that empty bucket. And so that this something would be truly important. At the same time, I suggest you look at your "empty bucket" differently.
This emotional anxiety, fatigue, confusion, dread felt as more real than anything I've ever felt before. It showed me not how things are, but how things are not — GreekSkeptic
The words are very reminiscent of modern books on psychology or psychiatry. This text is used in these books to describe the phenomenon they call "depression."
However, as I said above, I’d like to offer a different perspective: you do not lack a “will to live” at all — the very fact that you keep trying to fill that empty bucket proves the will is there, and it’s strong. This very striving already indicates that you have a will to live. You just haven't yet found what to fill it with—something worthy, meaningful, and important. This means that what you're describing above is consistent with the absence of something you believe is worthy in your "bucket," or with the fact that you haven't yet found it, but not with the will to fill that "bucket."
Your will isn't aimed at "living," but at "understanding why to live." It's truly a different kind of will—almost exploratory, almost scientific (congratulations, you're a philosopher!).
The thing is, like probably many participants here on the forum, and I myself, too, are searching for what to fill that bucket with. It's not scary to live in this search. This state is similar to “The Man Without Qualities” by Robert Musil (maybe I’ll write a separate topic about this).
All I can recommend to you now is not to panic and not to rush. Moreover, as you will gradually discover, even if you read 8-10 hours of various books every day, you will always find something surprising to behold the following day. Unless you forget how to be surprised.