Unfortunately, I came installed with a fairly broken evaluator of chances, which tends to consistently evaluate the probability of X happening to person P differently if P = me than if it isn't, all else being equal... and it's frequently true that my evaluations with respect to other people are more accurate than those with respect to me.

Then work towards the immortality of another. Dedicate your life to it.

You have not considered this thoroughly.

What are 28 mortal lives for one that is immortal? If I was asked to choose between the life of some being that shall live for thousands of years or the lives of thirty something people who shall live perhaps 60 or 70 years, counting the happy productive hours of life seems to favour the long lived. Of course they technically also have a tiny chance of living that long, but honestly what are the odds that absent any additional investment (which will have the opportunity cost of other short lived people), they have of matching the mentioned being's longevity?

Now suppose I could be relatively sure that the long lived entity would work towards making the universe, as much as possible, a place that in which I, as I am today, could find some value in, but of those thirty something individuals I would know little except that they are likley to be at the very best, at about the human average when it comes to this task.

What is the difference between a certainty of a two thousand year lifespan, or the 10% chance of a 20 000 year one? Or even a 0.5% chance of a 400 000 year life span? Perhaps the being can not psychologically handle living that much longer, but having assurances that it would do its best to self-modify so it could dosen't seem unreasonable.

Why should I then privilege the 28 because the potentially long lived being just happens to be me?

Only I can live forever. - is a powerful ethical argument if there is a slim but realistic chance of you actually achieving this.