I think part of my problem might be that I grew up with an entitlement complex as I was valedictorian, near perfect SATs etc. and I never did shit in high school.

Forget about that. Who you were at 17 means nothing. If you're 40 years old and still have to mention that you went to an Ivy League school, that's not a good thing.

It doesn't matter. No one knows who you are. The good news is that you don't have to fear embarrassment. When you're "the smart kid" you can't afford to look stupid. Now, you can. The world is built by ex-smart-kids who weren't afraid to look stupid.

I make $130k as a 21 year old and I probably put in 3 hours of real work a day.

Also irrelevant. I think it might be part of your problem. Forget precociousness and all that golden child bullshit, because any job that pays a 21-year-old (excluding overt nepotism/connections) that much money is going to be stressful as fuck (and, as I'll explain, I think cumulative stress is part of your problem). I think you might want to be moving away from the high-stress jobs. If you actually want achievements that you can be proud of, you won't get them in the high-stress professional gigs; you're better off downshifting to the middling level where you're still surrounded by smart people but no one's gunning to be a millionaire in 5 years.

I think people are starting to realize that I'm a bit slower than I could be.

"People" are just as insecure as you are. They're mostly focused on their own survival and advancement-- not sizing you up. They probably don't think about you at all. If you stay out of their way, their evaluations (except for management) of you don't matter at all. People won't fuck with you just because you're working below your potential; almost everyone is working below capacity. As long as it doesn't mess up their shit, no one will fuck with you.

What you have is impostor syndrome, meaning that you think you're worse than you actually are. The dirty secret of adult life is people are just making it up as they go along. You're not the only one who feels "fake", and the people who seem to have their shit together are just as shaky as you are.

I secretly spend 7-8 hours a day doing bullshit at work (reading online, games, etc).

Not uncommon. Don't feel guilty. Don't try to stop per se, so much as you should be replacing less useful activities with better ones. If you say, "I'm not going to [X]" you're setting yourself up for temptation. Better to find things that interest you more. Maybe you could learn new technical skills on the job. (Do your building-- that is, things where you need to own the IP-- on your own time and resources though.)

Your work environment is probably partly at fault, too. You probably spend so much time on bullshit because you can't get in the zone with all the distractions and interruptions and petty social anxieties that rule the day. You've probably noticed that people who actually want to get things done at work either (a) show up early, (b) stay late, or (c) spend a substantial amount of time outside of the office. Very little gets done in core working hours in the typical office, because most people are at 100% CPU on reputation management bullshit.

Here's something you probably haven't been told about the adult world. The reason most people hate going to work isn't the work itself. It's all the pointless social anxiety generated by cramped offices, interruptions, the constant need to modulate social status to precisely half a notch below one's manager, et al. People enjoy work itself; it's a deep-seated psychological need to feel useful and productive. It's being at work that gets them down, because full-time social climbing isn't natural or appealing to most people. The best way to become happier at work, perhaps surprisingly, is to work harder. That's not a platitude, though; it's hard advice to follow because you'll actually need something that motivates you to work hard.

I know that I've been given a gift and that I'm a fucking idiot for wasting it, but I've just become a chronic procrastinator and it sucks.

A very large number of people feel this way. One of the problems with Work in most jobs is that it conditions people to associate productive activity with subordination.

You're not a fuckup, but you've been poisoned with bad conditioning. It's not about "willpower" as some immutable trait of a person. It's about the fact that we're animals that respond strongly to our environment, rewards, and punishments. (See: Stanford Prison Experiment.) There's no point in feeling shame about this; it's just how our bodies and brains work.

I could be changing the world but instead I'm putting in the bare minimum

You're not ready to "change the world". You have to improve yourself first. I won't lie; it takes time.

Spin up a side project. Or replace your at-work videogaming with Coursera. Or take more responsibility at work. Try to move to another team. Just do something where there will be meaningful feedback from the world. What's the worst that can happen? If you get fired, it's still better to lose a job while on an upswing than when comfortable (the shock of getting fired while comfortable takes months to recover from; but if you get fired while you're actively working hard, it's much easier to bounce back).

Any advice on how you taught yourself to focus on tasks, build willpower, and get shit done would be helpful.

Just try to make each day better than the last. Install RescueTime. Don't expect miracles. Just work toward incremental improvements. Again, each day better than the last.

Although, I wonder if I really fucked my brain/habits up so much that I'll never reach my full capacity.

Unlikely. You can build new habits in a couple of months, and unless you were using a lot of drugs, you'll be fine.

Most likely, what has addled your brain is low-level but chronic social stress from the workplace. That shit sucks, but the good news is that it only takes about a month to recover (once you get to a better work environment). Practice meditation to build up your resolve (and don't expect results quickly; the contemplative path is a lifelong one).

By the way, "full capacity" is unattainable. You need down time. You need to spend some of your time playing video games and watching TV. Just focus on quality. I generally allow time for one high-quality TV show (e.g. Breaking Bad) but I sure as hell am not going to let myself spend 4 hours per day watching it.

My dad is also very similar in that he's smart enough to bullshit through life but he only works at 10-20% of his full capacity and he never completes anything.

Don't worry about your parents. Just focus on yourself. One of the most important subtasks of recovering your emotional health is to stop dwelling on details irrelevant to the task at hand. Observe, but never stew. Your father can take care of his own life; you need to focus on yours.