How do I get my insights to stick when people cling to assumptions?

You can influence now, which is real progress. What still eludes you is the hardest part: getting a true insight to hold against an assumption people are comfortable with. Facts alone rarely move a settled belief. People let go of an old idea only when a new one feels safer and clearer than what they had. Your job is not just to be right. It is to make the truth easy to accept. An assumption is usually protecting something: a habit, a fear, a past decision someone does not want to reopen. When you understand what the old belief is guarding, you can speak to that, instead of just piling on evidence. People change their minds when they do not have to feel foolish for having held the other view. Next time, before you present, name the assumption out loud and treat it fairly first. Then show, in one clear example, what it misses. Meeting people where they are earns you the right to move them somewhere new.