I was the eldest of the three, And to uphold and cheer the rest I ought to do—and did my best— And each did well in his degree. (lines 69-72)
The speaker believes that it's his responsibility to look out for his younger brothers. Maybe his attempts to keep their spirits up are the reason he was able to survive so long himself.
I had no earthly hope but faith, And that forbade a selfish death. (lines 229-30)
Even though he has basically lost all hope, the speaker doesn't want to die – allowing himself to waste away by refusing his food would count as suicide according to his principles, and that would be "selfish."