(65) Thou hast dealt well with thy servant, O LORD, according unto thy word.
Sometimes, we need to stop and reflect God's goodness; how He has indeed dwelt well with us. If we do a search in His Word, we will find that "He is faithful that promised" (Hebrews 10:23).
God doesn't merely expect us to keep His Word - He also keeps His Word.
(66) Teach me good judgment and knowledge: for I have believed thy commandments.
Believe - OT:539 'aman (aw-man'); a primitive root; properly, to build up or support; to foster as a parent or nurse; figuratively to render (or be) firm or faithful, to trust or believe, to be permanent or quiet; morally to be true or certain; once (Isa 30:21; interchangeable with OT:541) to go to the right hand:
Good judgment and knowledge come only from God. Before we learn them, though, we must believe them. Am I founded in the Word of God? Do I know that it is truth? How can I learn from something that I think is fallible? I must be fully convinced that God's Word is truth. I must have total faith that His promises are true. Only then can I make any progress in learning His judgment and knowledge.
A brief review of from Whom we're learning judgment and knowledge: "Who hath directed the Spirit of the Lord, or being his counsellor hath taught him? With whom took he counsel, and who instructed him, and taught him in the path of judgment, and taught him knowledge, and shewed to him the way of understanding?" (Isaiah 40:13-14)