The issue is that I don’t have an “ultimate” preference

Do you need one?

If you keep asking "why" or "what if?" or "but suppose!", then eventually you will run out of answers, and it doesn't take very many steps. Inductive nihilism — thinking that if you have no answer at the end of the chain then you have no answer to the previous step, and so on back to the start — is a common response, but to me it's just another mole to whack with Modus Tollens, a clear sign that one's thinking has gone wrong somewhere. I don't have to be able to spot the flaw to be sure there is one.

How could I convince my future self that my preferences are better than theirs?

Your future self is not a person as disconnected from yourself as the people you pass in the street. You are creating all your future yous minute by minute. Your whole life is a single, physically continuous object:

"Suppose we take you as an example. Your name is Rogers, is it not? Very well, Rogers, you are a space-time event having duration four ways. You are not quite six feet tall, you are about twenty inches wide and perhaps ten inches thick. In time, there stretches behind you more of this space-time event, reaching to perhaps nineteen-sixteen, of which we see a cross-section here at right angles to the time axis, and as thick as the present. At the far end is a baby, smelling of sour milk and drooling its breakfast on its bib. At the other end lies, perhaps, an old man someplace in the nineteen-eighties.

"Imagine this space-time event that we call Rogers as a long pink worm, continuous through the years, one end in his mother's womb, and the other at the grave..."

Robert Heinlein, "Life-line"

Do you want your future self to be fit and healthy? Well then, take care of your body now. Do you wish his soul to be as healthy? Then have a care for that also.