I am not a native English speaker myself (Danish), but the
overall gist of the poem was always easy to understand (for
me): It is, as you write, about the loss of innocence. Indeed,
the collection of poems it belongs to is entitled "Songs of
Experience". But the particular line I referenced is difficult
to understand, because the overall meaning of it obviously
seems like something positive ("witless woe was never an
issue") and yet you have these three negations - "witless woe"
has a negative meaning, "never" is a negation and "beguiled"
has a negative meaning - and multiplying three negatives yields
a negative, giving the whole sentence a negative meaning when
it should have the opposite?
The "hid from him my heart's delight" line, I understand as the
narrator not giving fully in to his feelings: He cries to be
sure, but yet holds part of his emotions back from the Angel and
therefore the Angel punishes him by fleeing. Perhaps the larger
meaning is one of religious devotion: You must surrender yourself
completely to God in order to obtain Bliss. 90% is not enough.
Morten