If you're in your early 20s I highly recommend The Little Black Book of Violence. Getting into fights is very stupid even if you're a professional. Especially with how social media amplifies NPC propaganda.As a young man, self defense training served me well against bullies and drunks, but as an older man I've certainly gotten out of practice.
That being said, I think it's useful to think about self defense about being able to resist/be a hard target against those people that are most likely to engage you.
As a youth, pre internet the likely threats that engaged me were generally thugs, drunks and bullies. Their means of engagement was physical intimidation and violence.
As an adult, and maybe even as a modern youth today what are the likely threats that will engage me? The public school system, the legal system, the healthcare system and other bureaucracy. I don't have a lifestyle that really puts me in range of likely physical threats, especially hand to hand unarmed combat. If I were to have physical threats engage me somewhere its likely going to be armed combat.
All of that aside, train for physical self defense but also train for procedural defense. Physical training raised my confidence as a young person but procedural self defense has raised my confidence as an adult. Knowing what your rights are, what you can say no to, how to navigate bureaucracies and interact with them will save your butt in non-violent conflict.
Dont' invest entirely in deterrence of physical violence.
There's also 2 schools of thought with self defense. Grey man versus the tough approach. Whichever you pick pick very wisely.