To be clear, Steering a trike is an exercise in push pull.
To be more comfortable in riding learn to lock the elbow you are pushing with to make steering less wearisome, should you come to a hard corner you may want to shift your butt on the seat ( like a snowmobile ) to plant your foot more solidly on the peg to handle the sensation of sliding off or tipping over?
Learn to look through a curve instead of concentrating on the road directly in front of you to avoid over correction, You may have noticed a two wheeler in a corner and a rider having to make allot of corrections in lean?
It tells me he isn't looking through the corner.
A mistake most people make with a trike Is lugging the engine.
( Keep your trike in gear where it is always pulling and it will reduce the sensation of head shake.)
Should you get head shake a touch of throttle or brake will stop it.
Try that and see if doesn't make a big difference, It's learning curve thing but is a telling difference in riding smoothness.
Should you continue to feel way more headshake than you think is normal take it back to your converter and ask him to check the torque on the stem nut, A trike has to have a greater torque on that nut than is recommended for a two wheeler.
For instance the book may recommend 40 lbs torque on the stem nut on my two wheeler, I might torque it to say 55 lbs? for a trike, a 15 lb. difference.
That is just a figure I pulled out of head but you get the picture.