Hmmmm. There are a number of people who have difficulty with one handed steering or even no handed riding. I personally don't understand this challenge. My thoughts go towards fit of the bike and how fit you are. Relying heavily on your hands and arms to support you can contribute to the challenge.

Once I started strengthening my core and getting the bike fit better, I can easily do left, right or no hands. For me it isn't getting to know where the shifters are, although I do know the general location, as much as having the core strength to be in any position and shift my weight and balance to the feet and saddle as needed. I noticed several months ago that I don't steer with my hands and arms but more with my torso by shifting body weight.

Another thought is that when I take my right hand off the handle bar, I use my core to support my upper body taking some of the force off the left hand on the bar. This allows me to control the bar with more finesse because there is less force on it.