And my soul shall be joyful in the LORD: it shall rejoice in his salvation. Psalms 35:9 (KJV)
Real joy does not come from things or happenings. The Lord Himself is the source of our joy. We can be joyful in the Lord which is a result of being in union and fellowship with Him. The Life and Presence of God in us is a cause of continual joy.
What the Lord has done for us in saving us always fills us with joy. We are filled with joy knowing, and assured that we have been saved, and are secure from all evil and harm.
Therefore set your heart and mind on the Life and Presence of God in you. Know that you are inseparably joined with Him, that He abides in you and you in Him and you will experience constant joy.
Let your heart and mind be always filled with the knowledge of His Salvation – that He has saved, healed, delivered and made you whole. This will keep you always full of the Joy of the Lord.