Disclaimer: Please read the post in full and I do not suggest you actually take such a dangerous jump.
The obvious answer and most difficult thing, since it is not always clearly available, is to try and work on similar situations at different heights in succession. If you are that afraid, try moving to something at a lower level and that is similar in features. Let me say this for my own sanity is that you should not attempt something like that unless you 200% know you can make that distance jump, in your sleep. I am not condoning this jump since the practicality that you find yourself in such an awkward situation on a roof top is far fetched, unless you are a bank robber?
I had a similar situation awhile back, where there was a jump from two leveled platforms (high to low.) With a decent distance between the starting to landing precision and a pit in between them that posed an imaginary threat. What I did was I practiced the jump a few times of the same distance and drop but with no gap in between and then being assured I cleared my mind, calming myself and then successfully did it. Again, don't take this jump so leisurely and I am not suggesting you actually do the jump at all since there are much more safer ways to practice.
Here is a much safer idea for your fear of heights, find a pool center or something of the sort that has safe conditions and multi-leveled diving platforms. Get used to the height in safety and have fun jumping off of that, but don't equate the landing in a pool to what is out there. In other words, don't get a false sense of security.
Hope this helps in some way.