Corners are relatively difficult. What you're looking for is the use of corner ramps. What your build lacks is a realistic curve (as far as I can tell from those pictures) so it's going to look odd. For a natural bend, you're going to have to give it space and build into the curve by edging closer and closer to it.
Trying bending the walls back and then up more randomly. This method (if done correctly) will leave holes in the wall that you can easily fix later with another 'rocky wall'.
Here are some pictures as an example:


Another method is to just crisscross ramps until it looks full, then use the same extra-wall method like so:

Lastly (this isn't entirely related to the curves but does help), your build seems to lack buildup to the walls. You can see something like what I'm talking about in the above picture as well. This would make the transitioning from straight to curved a lot easier.