(77) Let thy tender mercies come unto me, that I may live: for thy law is my delight.
Oh how we need the Lord's mercies to live! "It is of the Lord's mercies that we are not consumed, because his compassions fail not. They are new every morning: great is thy faithfulness." (Lamentations 3:22-23)
We never deserve God's mercies - but we especially do not deserve them when we refuse His law.
(78) Let the proud be ashamed; for they dealt perversely with me without a cause: but I will meditate in thy precepts.
Towards those who dealt "perversely" (wrest), the Psalmist committed them to God. Instead of meditating on how to "get even" or "get back," his thoughts were on God's precepts. How much better would my reactions be if I focused on God's Word instead of the problems others cause me?
(79) Let those that fear thee turn unto me, and those that have known thy testimonies.
Seems to go along with Psalm 119:63, "I am a companion of all them that fear thee, and of them that keep thy precepts." If our heart fears the Lord and we know God's testimonies, then we'll attract friends who also fear God and know His testimonies.
(80) Let my heart be sound in thy statutes; that I be not ashamed.
Sound - OT:8549 tamiym (taw-meem'); from OT:8552; entire (literally, figuratively or morally); also (as noun) integrity, truth:
Is my heart sound in God's Word? That will never cause shame before the Lord.