A long time ago, this happened to me too, but it is fixable! First, click WatermillTerrain.png and then middle click and then click "Edit with (Gimp/Photoshop)" and it will open up a page of either GIMP or Photoshop! Next, try your best to copy the terrain from the one you made before. After your done, make sure you DO NOT overwrite if, if you do, it can crash your game... (Learned that the hard way... Or maybe it was the material making that did that >.<) Anyways, after you "exported" the new height map, make sure it's saved under a new name so it doesn't overwrite the other one, (Example: feralTerrian.png)!
Try to copy the mask by doing the same thing I just explained above, but with the mask and not height map!

After you do this, your map should be amazing and if this helped, please put a link to the download and I'll see if I can download it!