Why can't I unwind when I'm alone with nothing to do?

I'm a 28M and I'm a firefighter, so I'm constantly being called to violent and traumatic situations to save people and property. I'm also in an amazing relationship with a 27F who loves me to death and I her. I'm financially stable, I'm in great shape, I have a good relationship with my parents, friends, and extended family, I have a nice house and a nice car, and I'm generally happy.<br><br>But for some reason, ever since I was a teenager, I can't relax when I'm alone. It's not that I hate being alone, it's that I can't unwind unless I'm doing something distracting. And it's not just when I'm alone, it's when I'm in a situation where I'm not actively doing anything. I can't go to the beach and sit in the sand. I can't go to a park and sit on a bench. If I'm at home by myself, I can't unwind unless I'm playing video games or watching a movie. If I'm without anything to do, I feel physically uncomfortable. And I don't know why. I'm not a high-energy person. I don't have ADHD. I've been checked for anxiety and depression. I don't have any medical issues. But I need constant stimulation or I can't wind down. I can't take naps. I can't read a book (because I can't focus). And it's getting in the way of my life.<br><br>In my line of work, it's normal to see things that most people can't imagine. Horrific things. And I feel like this constant stimulation is how I cope with the things I've seen and I'm afraid without it, I'll unwind into a puddle of PTSD and flashbacks. But I also don't want to be reliant on it anymore. And I don't know how. Any ideas?