Hi. I used to be scared of going upsidedown but it's not as awful or scary as you may imagine. On some loops, you don't even feel upsidedown completly. But I imagined them to be a lot worse than they were. I'd say inversions arn't as frightening as launches or vertical drops.
I recently advised a friend of mine who'd never ridden a thrill ride. he managed to go on the world's only ten-looping coaster and loved it. The only way to overcome your fear is to just get on a looping coaster. Trust me, you will feel totally secure and safe on a loop, barrol roll or boomerang. Hopefully, you'll enjoy it too.
Hope this helps.