This is probably not helpful, but I'll say it anyway.
I have never been persuaded that there is any human capacity, including fantasizing, that's evil in itself. Everything we are inclined to do is potentially helpful or harmful to us, depending on the way it is employed.
Many years ago, it struck me that fantasizing on the Divine is not the worst use of this particular human faculty. It really can be inspiring. I sometimes fantasize as to what the Beatific Vision might be for humans. Will we suddenly find that certain angels are the agent principles of, say, motion in the universe? Would we, confronting an angel, find that its very name contains every physical principle governing the work of atoms, light, and so on? Would we find that angel a cold thing or immensely warm and loving toward us? Would it welcome us into its particular domain? Would we marvel at its intense and gargantuan obedience?
Would we be able to inspect the interior of the sun, understanding (through participation in the Divine Intellect) every calculation explaining every motion of every atomic particle? Would we be able to "feel" the heat just as it is, but without being harmed by it? Would we "feel" the cold of outer space and enjoy it just as we might a cool summer shower?
Would we be able to view a growing tree from the inside, with no regard whatever to the time it took to see the motion, and would we perhaps be able to do that simultaneously with exploring the interior of a black hole?
Would we all, every one of us, be able to walk with Jesus at the same time, and could He address each of us as if we were the only person in the universe? I sometimes think about taking that walk, and think about what He might say and just how, exactly, He might express His love for me, and what He might tell me.
God is infinite. We will never exhaust His riches of wisdom, love and understanding. Eternity is not, paradoxically, long enough for us to do that. What will we see? How will we interact with other people? Will the old lady we saw for a few months in church before she died be able to present herself to us simultaneously at every human age? Will we then see, in her childhood self, why it is that God cherishes her as His child (and understand better maybe, why Jesus said what He did about "becoming as little children") Will we marvel at her beauty at 25, her dignity at 60 and her virtually unethereal presentation at 80? Will we then have a much better comprehension of humanness and its richness than what our own "timeline" view of things gives us?
That is definitely not to say contemplation of God is the only proper use of fantasizing. One must, after all, fantasize out a story in order to be able to tell it. It's how we use that marvelous faculty that counts. To me, it's a Divine gift, a very big one, and is part of what we are. Still, we are on this material world, and we must reconcile that gift with such mundane things as the need to clean the house, wash the car and go to work in the morning and give a good accounting of ourselves in that context.
Anyway, that's my contribution, whatever it's worth.