I think it's a natural human instinct to have a massive adrenaline rush in confrontational situations, but if watching a video on YouTube gets you worked up you probably have some issues you need to work through.

With no medical credentials at all, I'd say you have something deep inside you. Something vicious. You want to let out the Beast within, but you're stuck. Next time you get in a situation like this, say someone doesn't hold the door for you, or someone bumps past you in a rush, promptly punch them in the back of the head. This will allow you to break your fight hymen, with little to no possibility of getting hurt yourself, as you are striking them from behind.

Once you feel yourself break through that mental barrier, it will be like a physical weight off you. You'll be able to build your way up to eventually arm-barring the local maniac on the bus into submission and protecting those around you.

Who knows, you might even catch the eye of a young lady and get yourself a quicky for your trouble. Good luck

None of the 3 situations you described in the op require assertiveness on your part. People with severe mental illnesses on the bus are still going to have severe mental illnesses no matter how you respond to them. There is no amount of confident assertiveness that will cure someone else's mental illness. Avoidance is a perfectly normal response. If the guy was physically assaulting someone or breaking the law that's a different story, but it sounds like he wasn't.

With the debt collector, just hang up the phone immediately, why the hell do you need to say anything? The more you talk, the more worked up you'll get for absolutely no reason. Practice hanging up the phone the instant you realize it's a debt collector, and then block the number. Maybe have a friend call you and pretend to be a debt collector so you can practice.

As for getting nervous in a poker room, this is simply social anxiety, and it's about how you view yourself. There's no easy quick fix for this, you need to do work and get more comfortable in your own skin. This work could be psychotherapy, CBT, meditation, something else, all of the above maybe.

Exposure therapy is not really intended for any of these things you're talking about. Exposure therapy is designed for specific phobias, such as being irrationally afraid of spiders or dogs, or the inability to get on a plane because you're too afraid it will crash.

Watching random videos of psysical assaults or arguments just to get yourself worked up is probably the worst possible thing you could do.

I know how you feel. When I was in my teens and early 20s, I used to have a Ka-Bar knife with me at all times, and I really wanted someone to break into my house just so I could stab the **** out of them and unleash all my rage in a legally justifiable way.

I'm 35 now and that's never happened, because the odds of it happening are rather small. Even if it did happen, would it have helped? Probably not. I think the best course of action for you is to take some initiative to get to the root of your rage issues as opposed to waiting around for someone else to give you the chance to outwardly express it, while you slowly kill yourself with stress and anguish.

Or at least start doing some kickboxing at a gym or whatever.

Exposure therapy in the OP's case is only going to lead to irrational fears and cognitive biases toward the "avatar" of the aggressor in the videos. He already seems hypersensitive with a touch of paranoia. I'm sure we all have or know those people we know on social media who have been doing nothing for the past couple years except watching & sharing videos of cops killing civilians, "BLM" attacking innocent people, drug cartel snuff/torture videos. All they're doing with that is feeding the narrative in their head of why you should be afraid of a particular group.

As cruel as it may sound, the first step to peace is to realize how insignificant your time/existence is in this world. OP's main problems seem to come in public situations where he feels that confrontation is about to transpire between strangers. Most of our interactions with strangers away from a poker table only last mere seconds. 80,000+ seconds in a day. These people won't remember you or the 5 second interaction that they had with you even if you asked them about it a few hours later.

No one is out to hurt you.

Google Operant Conditioning

This.

Martial arts training has helped adjust my fight/flight response. It takes time to reverse that stuff though.

To be honest, I really wouldn't get involved with crazies or seek trouble just because I knew some martial arts. After all, it's supposed to be self-defense unless you're fighting professionally or a murderer.