I have struggled with paranoia from age 11. I had terrifying real life abuse triggering events. I am always hyper vigilant and have PTSD. I have very serious paranoia and phobias. However I am rarely lost entirely in them. Most of the time part of me is questioning the situation and at times even fighting with the paranoia and rest of my brain. It can be exhausting.
The few times I have 100% believed the paranoia have been frightening. I fully believed what the terror was telling me and behaved as if I was in danger or under attack. Try explaining to a family member you think he is means you bodily harm when he doesn't.
My therapist said the questioning part of me needs to be listened to. That is the rational and realistic part of me. But it can be difficult when I am triggered, paranoid, terrified, and want to crawl under my office desk and hide. Grounding helps me to distract from the paranoid thoughts. You might check out the PTSD forum on PC. Even if you don't have a formal diagnosis, I am over there. I find a lot of good info on handling stress, panic, and paranoia.