It takes a lot practice.
Letting go of wanting to control, and being able to tell yourself that everyone is separate, everyone has their own mind, their own thoughts, their own feelings takes a lot of practicing. But it is freeing too! Because you get to where you know that you have no control whatsoever over the other's responses, so you can relax. Life becomes more interesting as you just let things happen without trying to control them, or to control the outcome, or to know the outcome. I used to say that I wanted to know what was 'down the road before I got there'; and that was the phrase I used with myself when I noticed that I was spending a lot of time thinking about what might happen. "Here I am again, wanting to know what's down the road when I can't possibly know!" Sometimes I would have decided something was going to go a certain way, other times I would stew about how something might go. Either way, I had no control over it, so when I practiced reminding myself of that, I began to feel relieved in response.

You can't know... what you don't know. Freedom!