Welcome to s101. Could the handicap be causing you to tremble ?? If not, it could be many things.
The most common reason for being unsteady is just not knowing what you're doing. In other words, you're not sure where you hand/holding the brush is supposed to go and it takes longer, thus making you jittery. I worked along side of a guy 40 years ago whose hand shook like crazy, until his brush hit the truck, board, whatever...... then he went like it was nothing. Its the assurance you build up over time in most cases. Practice will slowly take that away. I don't use a mahl stick all that much, but I do use my pinkie quite a bit. Left over from my fine-arts days. I have several mahl sticks from metal, to wood to anything I can rest my hand on, including my other hand propped up on elbows.
You'll get the hang of it.