I opened my mouth and panted - All this is the language of deep emotion. We breathe hard under the influence of such emotion; we open the mouth wide, and pant, as the ordinary passage for the air through the nostrils is not sufficient to meet the needs of the lungs in their increased action. The idea is, that his heart was full; that he had such an intense desire as to produce deep and rapid breathing; that he was like one who was exhausted, and who “panted” for breath. Compare the notes at Psalms 42:1.
For I longed for thy commandments - The word here rendered “longed” occurs nowhere else. It means to desire earnestly. See the notes at Psalms 119:20.