The term joy is often misunderstood by modern Christians because of the way it is defined and used in our culture today. Joy is often considered a synonym for happiness, which in modern use is understood as more sentimental than substantive.
Belief is not circumstantial but is a gift of God’s grace and must be received as such. You cannot will it to be, and it will never be on your terms. Try harder and do better, exhaust your efforts on the treadmill of works, if you please, but the gift of grace is just that, a gift so that no one may boast.