I feel a lot of pain but it's not physical, it feels really awful, last night I was trying to sleep but I just couldn't because the moment I closed my eyes and stopped using my phone, I started having very painful thoughts, my heart was pounding, thoughts about what's wrong with me?, what am I going to do with my life?, how defective I am, a lot of self hating thoughts, a lot of existential and philosophical thoughts, i kept thinking what mental illness do I have and kept browsing subs like r/AvPD and this subreddit, I don't know what to make of last night, it was extremely painful but it wasn't physical pain, is this what you call anxiety? I feel like it was far worse than that, at one point i really wanted to shout but couldn't because i have neighbours and thin walls, i probably wouldn't screamed even if i was alone cause I'm very socially anxious and would be embarassed to. I finally slept at 5 am and woke up at 10 , i grabbed my phone soon as i woke up because I'm addicted to and have nothing else to distract or curb the pain, how do you guys describe this "mental pain" is it just anxiety?, fear?, idk.
I could be going through mental anguish at some point of time and you wouldn't even know it from my face, I'd just look like a normal gut sitting over there, that's how good I'm at hiding it, what else am I supposed to do? squirm like A child? nah, that's too embarassing to do. 
When someone asks if something's wrong with me or if am i doing well, I'm never able express what I feel, I'm very bad at expressing and putting thoughts into words