The great difference between joy and fear is simple: Joy can take place in all three ‘time zones’ — past, present, and future. Whereas fear is only of what is going to happen (to us) in the future.
We cannot be afraid of the past. If we are afraid of something in the present it is really because of what we fear might happen in the next moment, hour, year…
On the other hand, one can have a joyful memory, a joyful now, and the joy that comes from expecting something great in the future.
Put another way, fear can only live in the abstract. Joy lives in both the abstract and the concrete.