Making up the Tralfamadorians and their philosophies can be seen as Billy's way of coming to terms with the things that he cannot understand, a way of silencing the dissonant thoughts in his head. He was shaping his thoughts so that he could live in a world where he understands how it works. It was his way of escaping the things he has experienced, or perhaps also a way of making sense of the things he has experienced. What this means to our understanding of the Self is that our past experiences shape our present Self. And in the same way, our past experiences shape our future Self. The Self is the totality of our past, present, and future experiences. How we define our Selves now is a product of all our experiences in the past and in the present. The experiences of the people around us also shape our Self. The experiences of the people who came before us shape our Self. History shapes the Self. History is a part of the Self. And if history shapes the Self, to a great extent, as the Tralfamadorians argue, there is no free will.

Kurt Vonnegut also explores the idea that people have no free will. They cannot...

Any attempt to change the past, present and the future is futile since history has forced people to be in a given moment. In this way, fate exists and it is what shapes the Self and life.
This manner of looking at life and at the Self-presents us a different view of life. Instead of being active participants in the course of our lives, we are but passive participants who have little control on how our lives turn out. Our circumstances in life happened not only because of the decisions we have made along the way; it happened because of all the things that have happened before us, long before we were even born. Our circumstances have been defined by the collective experiences of all the people around us, of all the people who have come and gone before us. Our present circumstance is a result of the lack of free will. It is a result of fate. We have been thrust with the kind of life that we have. And since we little say in what happens to our lives, it only seems sensible to accept the things that come our way.

Reference

Vonnegut, Kurt. Slaughterhouse-Five. Ebook. Dell: 1991.