In this verse, the Lord is praised. In Psalm 145:12, it was his kingdom,
but the first word of Psalm 145:13 is your kingdom.
The greatness of God is sung about, with lavish formulations expressing that his kingship is forever.
The sentence which is found in the Dead Sea scroll of this psalm, and which is reflected in some translations, says that the Lord is faithful to his promises and trustworthy in everything he does. You can count on him in word and deed. Direct praise to the Lord and extolling his praises follow interchangeably.
Some translations place the extra sentence as the beginning of Psalm 145:14.
13 Your kingdom is an everlasting kingdom, and your dominion endures throughout all generations. [The LORD is faithful in all his words and kind in all his works.]