Why did he say "Ki Chasdecha l'Neged Einai"?
Radak: My heart did not veer from You because Your Chesed was always in front of my eyes.
Malbim: He explains how he was pure in his deeds. (a) The Chesed that Hashem did for him was always in front of his eyes. Due to this, he served him amidst Hakaras ha'Tov, and because he depends on Him to receive His Chasadim constantly. Also refer to 26:3:2:2.
What is the meaning of "v'His'halachti ba'Amitecha"?
Radak: This is like "Es ha'Elokim His'halech Noach" (Bereishis 6:9). He went in Hashem's ways b'Emes and with a straight heart.
Malbim: This is the second way he became pure in his deeds. He recognized the truth of Hashem's grandeur and exaltedness. This is a higher level, to serve amidst Yir'as ha'Romemus.
