Lamentations 3:22–23 was written from inside Jerusalem's complete destruction — the city burned, the people exiled, the covenant seemingly broken. The author makes no argument about why it happened. He holds onto one thing: "It is of the LORD'S mercies that we are not consumed, because his compassions fail not. They are new every morning." The morning renewal of mercy is not a feeling. It is a theological claim made from the worst possible position. Feeling like a failed spouse often means surviving one morning at a time, and this verse was written from exactly that position.
Jeremiah 29:11 was written to people in exile — people whose original plan had been canceled, whose life structure had collapsed. "For I know the thoughts that I think toward you, saith the LORD, thoughts of peace, and not of evil, to give you an expected end." The Hebrew word for "expected end" — tiqvah — means hope, something worth expecting. God's plans for a person are not canceled by the collapse of a marriage.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.