Trust with a new stepparent is built like a friendship: slowly, and mostly over small, ordinary stuff.

Nobody walks into a house already trusted, and a title does not change that. The pressure in that situation is usually to feel something on a schedule, which is not how any relationship works, including the ones you chose. What actually builds is slow and unremarkable: a ride, a show you both follow, a question about your day that gets a real answer. You get to set the speed, and going slowly is not a failure.

If it feels stuck, look for one ordinary thing rather than one big conversation. Ten minutes in the car counts. And if it stays hard for a long while, that is allowed too.