Fear has an unusual relationship with time. The thing we dread may be in the future, but the mind can make us begin living through it now. Seneca is not saying that danger is imaginary. He is noticing how anticipation can add another layer of suffering before we know what will actually happen. Imagination is useful when it helps us prepare; it becomes costly when possibility begins to feel like memory. Returning to what is known can give the future back some of its uncertainty. What worry are you living through before you know whether it will happen?