I'm not a tile guy but I did the same thing and I'm learning it was "wrong".
I put Blue rock AND durarock, obviously I now have the same problem as you. However, my shower has been tiled already, I have a bigger gap on the outside edge that's too big for grout. That will look worse.
Get a piece of 3/4" pvc quarter round (lowes), paint desired color, put WP adhesive on it and put on edge. Fills nicely and looks good too.
Easy, good looking fix.
If you haven't tiled yet, I'd remove cement board and drywall then reinstall cement board to stud.
Paint on 2 coats of WP membrane.
Let dry, then tile.