I'm having difficulty in understanding the following example sentence. Would you please help me? If it were something like "I've done my best and all there is to do again", then it would have made sense to me.

to do

to do : necessary to be done *I've done my best and all's to do again— A. E. Housman*

[M-W's Col. Dic.]
Let's look at the entire poem:

Yonder see the morning blink:
The sun is up, and up must I,
To wash and dress and eat and drink
And look at things and talk and think
And work, and God knows why.

Oh often have I washed and dressed
And what's to show for all my pain?
Let me lie abed and rest:
Ten thousand times I've done my best
And all's to do again.

I think he's saying that despite his somewhat boring daily routine, there is little or nothing to show for it. The last sentence probably means something like: ..."only to have to do the same thing, over and over again."
Houseman was English, and this is an English way of expressing that all of it is there to be done again.