I have said. Some codices, with Septuagint and Vulgate, read "Thou hast said". Compare Psa_89:19. The words of Ethan, reminding Jehovah of His covenant with David.
Mercy = Lovingkindness, or grace. Note "Mercy" (Psa_89:2); "covenant" (Psa_89:3); "seed" (Psa_89:4); repeated below (verses: Psa_89:19-32 and verses: Psa_89:33-37).