I'd say give it some time. You probably played a very similar neck on your Martin X and lately just got used to a thicker neck. Here's a good chart showing the different Martin neck profiles;
My cs OM-18 has a low oval neck. It's thicker than the neck on my Road Series and I go back and forth like it's nothing. I've also played the mod V neck and didn't even notice it.
I would hate for you to sell this guitar, have a custom d-18 made and then think ''this sure is a lot like the other one''.