Joy is a choice.  I know I personally struggle that when I'm feeling negative emotions, those emotions are my entire focus.  This makes it almost impossible for me to feel Joy.  And here God is telling me to choose to be joyful.

Let the negative emotions you feel - anger, frustration, sorrow, pain, hurt, betrayed, mistrust, discouragement, disgust, anything else...  Let these negative emotions be reminders of God and who He is.  Let them remind you to go to Him.  Tell him what you're feeling and why. Ask Him questions and prayer requests, vent to Him.  And then...

Choose to be joyful.  Even if you no other reason in the world... At the very least, you can be Joyful in God your Savior.  You can be joyful that He sent His only Son Jesus Christ to die on the cross for your sins.  You can be joyful that Jesus rose again and that He lives in you. 

"Though the fig tree may not blossom, nor fruit be on the vines; Though the labor of the olive may fail, and the fields yield no food; Though the flock may be cut off from the fold, and there be no herd in the stalls - Yet I will rejoice in the Lord, I will joy in the God of my salvation.  The Lord God is my strength; He will make my feet like deer's feet, and He will make me walk on my high hills. "
                 - Habakkuk 3:17- 19.