Since we have joy (a state) we can choose happiness (a reaction). God would have no reason to speak about a merry heart, if we could not change it. Think on heaven (future), not events (past).

“If in this life only we have hope in Christ, we are of all men most miserable.” (‭‭1 Corinthians‬ ‭15:19‬)

God’s Word changes lives.