"How could I how should I..."

Yesterday morning in the Clare archive I was examining Pet MS D20, which is simply a (blue) cover of a quarto exercise book that belonged to Clare son.  It's dated 1841.  I copied one of the poems scribbled thereon (it did not seem familiar), but when I checked I found a slightly different version is part of Child Harold from, of course, 1841.  Below is my copied version from yesterday.

How could I how should I — that loved her so early
Forget when I've sung of her beauty in song
How could I forget what I've worshiped so dearly
From boyhood to manhood and all my life long
As leaves to the branches in summer comes daily
& blossoms will bloom on the stalk & the tree
To her beauty I'll cling & i'll love her as truly
& think of sweet Mary wherever I be

Child Harold

(lines 485-492)