Albert Barnes Commentary Psalms 108

O God, my heart is fixed - Prepared, suited, ready. See the notes at Psalms 57:7. In Psalms 57:7, this is repeated: My heart is fixed; O God, my heart is fixed: indicating that there might have been some doubt or vacillation caused by the circumstances existing then. The repetition would relate to that, as if the psalmist had been unsettled and wavering for a time, but was finally firm. In such circumstances, it would not be unnatural to repeat the assertion, as if there were no longer any doubt. In the beginning of a psalm, however, where there had been no previous expression or feeling of doubt, as far as it seems, there would be no propriety in repeating the assertion.

I will sing and give praise - See the notes at Psalms 57:7.

Even with my glory - This is not in Psalms 57:1-11. It is literally here, truly my glory. In Psalms 57:8, however, the expression, Awake up, my glory, occurs, and this seems to correspond with that language. It means here that it was his glory—his honor—to be employed in this way in giving praise to God. It was worthy of all that was elevated in his nature, of all that constituted his glory, of his highest powers. At no time is man employed in a more noble and lofty work than praise.

Awake, psaltery and harp ... - This is copied without change from (Psalms 57:8).

I will praise thee, O Lord ... - This is taken from (Psalms 57:9). The only change is the substitution here of the name יהוה Yahweh for אדני 'Adonāy. Why that change was made is unknown.

For thy mercy ... - This is taken from Psalms 57:10. The only change is in the expression above the heavens, instead of unto the heavens. The sense is essentially the same. The particular idea here, if it differs at all from the expression in Psalms 57:1-11, is that the mercy of God seems to “descend” from heaven upon man, or “comes down” from on high.

Be thou exalted ... - This is taken from (Psalms 57:11). The only change in the Hebrew is in the insertion of the word "and," and thy glory above all the earth.