Visualizing

I find it hard to control what I visualise. For example if I try to visualise that my boyfriend is having fun on his journey in South America, then all of the sudden he gets abducted, beaten or robbed in my visualisation. I have faith in the fact that these horrible things won’t happen to him, but they still pop up inside my head, when I try to visualise that he’s okay.
I have that kind of problems almost no matter what I try to visualise, and even when I’m listening to a guided meditation. What happens most often is, that the happy faces turn into scary, screaming and horrifying faces.
What can I do to gain control over, what I see when I visualise? And is it normal to have this problem?