Adam Clarke on Hebrews 12:5

And ye have forgotten - Or, have ye forgotten the exhortation? This quotation is made from Pro 3:11, Pro 3:12, and shows that the address there, which at first sight appears to be from Solomon to his son, or from some fatherly man to a person in affliction, is properly from God himself to any person in persecution, affliction, or distress.

Despise not thou the chastening - Μη ολιγωρει παιδειας Κυριου· Do not neglect the correction of the Lord. That man neglects correction, and profits not by it, who does not see the hand of God in it; or, in other words, does not fear the rod and him who hath appointed it, and, consequently, does not humble himself under the mighty hand of God, deplore his sin, deprecate Divine judgment, and pray for mercy.

Nor faint - Do not be discouraged nor despair, for the reasons immediately alleged.