Just to be clear (as someone who won't do rollercoasters that go upside down), you don't actually go upside down on FoP - it's the video that gives that sensation.
I think what helped me feel more comfortable with FoP relative to a rollercoaster was firstly that I know that it's only simulated, and secondly that one of the things that makes me fearful of rollercoasters is the feeling that they are "out of control", while the banshee on FoP is definitely "flying" (acrobatically) rather than... careening. But that's not going to help if the sensation of movement triggers motion sickness or the like, of course.
I think what helped me feel more comfortable with FoP relative to a rollercoaster was firstly that I know that it's only simulated, and secondly that one of the things that makes me fearful of rollercoasters is the feeling that they are "out of control", while the banshee on FoP is definitely "flying" (acrobatically) rather than... careening. But that's not going to help if the sensation of movement triggers motion sickness or the like, of course.