The meaning of the quote, "We're oft to blame and this is just too much proved that with devotion's visage and pious action we do sugar o'er the...

This quotation is the advice Polonius gives to his
daughter Ophelia as he, the king and the queen plan to stage a "chance" meeting between
Hamlet and Ophelia.  They hope to determine if Hamlet's insanity has been caused by
Ophelia's rejection.

Polonious demands that his daughter
pretend to be reading a book of prayers even though she is waiting to encounter Hamlet.
This is the "devotion's visage and pious action" from the quote.  The intent, "to sugar
o'er the devil himself," is for Ophelia to appear godly and innocent as she
waits.

Truly, all three of the adults here could be talking
about themselves.  They appear to be good on the outside, but inside they are all guilty
of plotting and manipulating.