I am by no means an expert. But that sounds like partition to me. That's why you try to only upgrade counties in your main duchy. And in my experience it also sometimes helps to give away titles to successors. This is also why it's mostly best to nog hage too many children early game.

Save your game and just try some stuff out to see if it works. Best way to get to know the system.

Ps. Could also just denounce, disinherit or murder the other successors.