Try a Leaf! It has a pivoting head and makes for a really comfortable shave on the bikini line. I had a Merkur before and felt the same as you and switched back to disposables. No matter how careful I was I couldn't get as good a shave as I wanted in that area. I've had the Leaf for a year now, and it was a game changer. I've not even nicked myself once with it.

Edit: Just don't expect much from the Leaf blades. They're okay for like one shave before they dull. Grab some Astras or Nacets.