The problem here is that the people who say this fail to comprehend the insane length of time that is a hundred years and also don't understand the meaning of 'eternity'.
Sorry for being rude but I noticed lately number of people writing: church this, or church that, while their sole knowledge about churches comes from anime or webnovels.
It requires a special amount of arrogance and ignorance at the same time to think that you are the first person who thought about eternal life or to claim that church "fails to comprehend". Church is at least 1994 years old. And through all those years was sustained by multitude of scholars. Church managed to preserve knowledge of roman empire and build on it. Church was and is filled with brilliant scientists, inventors, philosophers and theologists. Those two last groups tacked problem of eternal life many times in last almost 2000 years. I can assure you that church knows the most about that topic.
But before church was established in year 30 AD there were civilizations and religions who also tried to tackle that problem.
That said please read myth of Tithonus and Eos. It is older than Homer who lived in 8th century BC.
It describes dangers of just eternal life.
Ok enough of my rant. I do not have anything against you specifically.
It is just the multitude of topics that are similar in underestimating wisdom and knowledge amassed by church over the years.
To answer your question.
What about you? What grand goal will keep you from killing yourself out of boredom.
To function as an ethereal being I would need a way to change my appearance, to change my identity every four to six decades.
I see it as extremely important in age of electronic identification and government databases.
Too long lifespan would be suspicious enough to grant you eternal life chained in some secret laboratory as a lab rat so that the wealthy elites could take your gift for themselves.
In face of eternity, changing yourself to a baby to get new ID and waiting those few years until you are 18 (again), to be able to continue your mission is a small trade off.
In past you could retain your body and just move to different town or country. You could go and explore new lands and than go back and claim you are your own child. It was not without fuss but was easier done than in today's digital age.
(accidentally one of the stories I write right now has a character that was changing towns for centuries until digital age came and now has problems to adapt)
What would be my goal?
Science!
With eternal life I would finally have time to learn all the things I want to learn!
All disciplines are interesting and i would like to learn them all. But I would concentrate on physics and engineering, to secure mode of transportation, so I could leave earth, before it will be swallowed by sun (which is predicted to happen in few years).
Vision of floating in the vast emptiness of space without any space ship is both boring and terrifying. Not to mention that without way to leave earth's surface I would be stuck being fried on the surface of sun for billions of years.