Rashi: You fulfilled Your word to him. This is like "v'Shamar Hashem Elokecha Lecha" (Devarim 7:12). Here it refers to the Chesed that He promised to David via Nasan ha'Navi.

Malbim: Malchus Beis David is founded on Emunah and Chesed. What is from Chesed does not depend on merit and people's actions. However, it is possible that later someone else will find grace [in Hashem's eyes] and the Chesed will be transferred to the one who is most proper for it, like was with Sha'ul and David. What Hashem does via Emunah, it is a guarantee according to the recipient's merit and being proper to receive it. It is limited according to the person's deeds. Both of these applied to David. Hashem raised him to kingship via Chesed, but it was because David was worthy of it. Even if someone else would be found with more merit, he would not merit to build a Bayis Ne'eman (permanent kingship), rather, another reward. Since the Chesed to David was due to his righteousness, it will never depart.

Malbim: Emes is true knowledge and belief in matters of investigation. Tzedakah is good deeds Bein Adam l'Chavero. Yishras Levav Imach includes all good Midos to go in Hashem's ways - just like Hashem is merciful [you should be merciful?], and to accept everything with love. Since the Chesed to David was due to his deeds, you gave to him a son who sits on his throne it will never depart.