Anxiety alone is a perfectly healthy emotion. It's when it happens by itself, all the damn time that it's considered a problem. And boy, it's been a problem for me. Most people understand the regular symptoms: pounding heart, sweaty palms, racing thoughts, shortness of breath, but the lesser known symptoms are often the scariest.
One of these lesser known symptoms is called Derealization. It's considered an alteration in the perception or experience of the external world so that it seems unreal. (as defined by Wikipedia.) In my experience, it makes me feel like the world around me is suddenly unreal, like a dream or a movie I'm watching. It's thought by psychologists to exist as your body's way to cope by simply tuning out the real world - and it does NOT mean you're crazy or anything is wrong with your brain, it just means you need to learn appropriate coping skills.
"I would just be walking, talking with friends, and then suddenly experience the whole world flattening into a 2D movie screen in front of me and being stuck in that trance for what felt like 5-6 seconds."
It would often happen when I was in wide open public spaces like school or the mall. I would just be walking, talking with friends, and then suddenly experience the whole world flattening into a 2D movie screen in front of me and being stuck in that trance for what felt like 5-6 seconds. It would always completely jolt me and trigger all those more common anxiety symptoms I already mentioned, and would leave the world around me feeling unreal and dream-like. This would happen sometimes 5-10 times a day, sometimes even all within 10 minutes like waves leading up to a full panic attack.
I eventually learned a few ways to handle these events. The best way I learned to handle these symptoms was to do legitimate Reality Checks. Wiggling my toes and feeling them press into my shoes. Counting my breaths. Picking an object and describing as many things I can about it. Psychologists might call this Mindfulness.
However, since in my brain I really associated derealization with dreaming, I found that doing Dream Checks* worked really well for me. For example, In a dream, if you look at a clock, look away, then look back again it will never say the same time twice. When I'm feeling like what's around me isn't real, I'll use checks like this to confirm I am, in fact, awake, and then I will begin using mindfulness to reconnect. (Although I don't experience derealization nearly as often or intensely as I used to, I still use this coping mechanism when I'm feeling uncomfortable.)
*PS: these also increased my chances of lucid dreaming, since when you practice these when you're awake, you'll start doing it in your dreams. Just like if you do it while you're awake you'll know you're in fact, awake, if you do it while dreaming you'll also know you're dreaming. This proved a huge upside, but use with caution as lucid dreaming can be scary at first.