I've not much to add to the very good feedback you've been getting. And I think you already know what the answer is.
I hope you can find it in yourself to bite the bullet and tell fantasy guy you don't want any further contact with him. You can tell him you're flattered by the attention, but that you are very satisfied with your relationship and life right now, and that the continued correspondence is inappropriate for you.
It always feels good to have a mental "out" for when things get tough, but that doesn't mean it's healthy for you. Deal with your present relationship--the good and the bad--on its own terms. Comparing it to fantasy will not help you.
Hugs,