Albert Barnes Commentary Psalms 118:28

"Thou art my God, and I will give thanks unto thee: Thou art my God, I will exalt thee." — Psalms 118:28 (ASV)

You are my God, and I will praise you — This is the language of the author of the psalm—his solemn profession before the sanctuary and the altar; his response to the priesthood.

In (Psalms 118:27), they had declared that “Jehovah alone was God.” To this he now replies that he acknowledges it; he recognizes him as the true God and as his God; he comes to praise him; and he professes his purpose always to exalt him as his God.

You are my God, I will exalt you — Repeating the solemn declaration that Yahweh alone was the God whom he worshipped, and that it was his purpose always to magnify his name.