This is an age-old question that has been answered in many ways depending on one’s wants and fears. It seems the driving concern is actually more about punishment in the after-life as much as it is about a blissful eternity. Will the guilty be punished? Will there be an end to misery? Will there be a reward for doing good?
Some see life as a succession of re-incarnations. The quality of one incarnation is determined by the previous one until that life’s essence has been lived out to some standard of perfection. The ultimate goal is to merge one’s essence with the greater essence that is beyond the physical realm.
In this view each life is eternal, but the quality of life will vary. I don’t think it answers the needs behind the question.
Some point to a church’s teachings, rituals and authority. If one accepts what is taught, undergoes the required ceremonies and satisfies the leader’s demands, then one can have eternal life. For this to be acceptable the church has to be a reliable authority, unchanging and consistent. When the teachings change and the humans in authority in the church prove to be untrustworthy, then the assurance of a happy after-life becomes unreliable as well.
Another common teaching about the after-life is once saved, always saved. In this teaching one’s assurance of eternal life is based on experiencing God/Jesus in one’s heart. It is usually expressed as an ecstatic experience. A problem arises when someone obviously backslides into an evil life. Then the explanation that the devil can deceive a person with a false saving experience makes all other saving experiences suspect.
So, getting back to the original question, I would encourage that we reframe it in a different way. Take out the ideas of rewards or punishments. Whether it is this present temporal life or an eternal life, it will be worth living if we have learned how to love others and ourselves. Then time won’t matter for even now life is eternal.