Thy testimonies - Thy law; thy revealed will; the revelation which thou hast given considered as thy solemn “testimony” as to what is true and right.
Have I taken as an heritage for ever - As my inheritance; as my property; as that which I consider to be of real and permanent value. The Hebrew word used here - נחל nachal - means to receive as a possession; to acquire; to possess as wealth; and then, to inherit. It is usually applied to the possession of the promised land as an inheritance. Here it means that the law of God was to him as such a possession. He regarded it as one does a rich inheritance. He chose it as his portion above all things else.
For they are the rejoicing of my heart - My happiness is in them. I find constant comfort in them. See Psalms 119:77, Psalms 119:92. Compare the notes at Psalms 1:2.