As a child, I had some ability to play pianissimo, at least in slow passages, but was less comfortable in piano. I thought piano ought to be easier than pianissimo, but it wasn’t.
Over many years, I worked on soft playing, trying out my own and others’ ideas; and I did improve somewhat, to the point of getting a compliment on my pianissimo from someone whose judgment I trust. This series of postings presents what I’ve learned so far. May you find it useful!
By the way, I’m writing on the assumption that your piano’s action—the key-mechanism—is properly adjusted. Playing a piano that’s “out of regulation” is like driving a car with a loose steering wheel.
I. The fastest way to good soft playing: Just do it!
Another fast way: Imitate someone who does it well.
It’s easy to make fun of such exhortations—like “Just say no!,” they can sound simple-minded—but I think they help some people a lot, and many people at least a little. A flutist I know says that hearing James Galway makes her play better for the next 45 minutes. My own practice improves after I watch Carlos Kleiber conduct:
II. Basics of Piano Dynamics
Piano sound is created by hammers striking strings. Loudness is controlled by the speed of the hammer: faster makes louder, slower makes softer.
Comparison: If two cars are to travel the same distance and cross the finish line together, the one that goes slower must start earlier. If two piano keys are to travel the same distance and sound their notes together, the one that plays softer must start earlier.
III. Imagining
Sitting away from the piano, imagine playing softly. If this makes you tense, release the tension systematically. Do this daily until the tension doesn’t appear. Next, do the imagining while sitting at the piano, but with the instrument completely closed. When you can do this with no tension, do it with the lid open but the keyboard still closed. Next, with keyboard open and hands in lap. Then hands on keys, but not playing. Finally, when playing.
IV. Psychology
Identify with soft!
to be continued . . .
On a different subject: Should all playing be playing by ear? Even if we read from score, should the process be, not eye to hand (as so many of us have been taught), but eye to ear to hand? Your observations welcome! 
