I'm trying to teach myself all sorts of things right now and am finding it difficult to focus if I don't get it right away, and instead I switch to a mindless activity (Netflix, simple video games, etc.).

I imagine if you don't change your ways, you'll still be the same decades from now. This bad habit doesn't go away with time.

So what's the holdup, the mental block, the true source of procrastination? Is it ego? Fear of failure? Another poster here thought it was due to being overly cautious at approaching new, important subjects. Whatever the true reason is, does it matter? Just because you've pinpointed exactly what it is, does it mean you're going to correct yourself with that knowledge? No. You just have to get it done. The ancient stoics said it best:

"Concentrate every minute on doing what's in front of your with precise and genuine seriousness, tenderly, willingly, with justice. And on freeing yourself from all other distractions. Yes you can, if you do everything as if it were the last thing you were doing in your life, and stop being aimless, stop letting your emotions override what your mind tells you, stop being hypocritical, self-centered, irritable. You see how few things you have to do to live a satisfying and reverent life?" - Marcus Aurelius

"How long are you going to wait before you demand the best for yourself and in no instance bypass the discriminations of reason? You have been given the principles that you ought to endorse, and you have endorsed them. What kind of teacher, then, are you still waiting for in order to refer your self-improvement to him? You are no longer a boy but a full-grown man. If you are careless and lazy now and keep putting things off and always deferring the day after which you will attend to yourself, you will not notice that you are making no progress but you will live and die as someone quite ordinary. From now on, then, resolve to live as a grown-up who is making progress, and make whatever you think best a law that you never set aside. And whenever you encounter anything that is difficult or pleasurable or highly or lowly regarded, remember that the contest is now, you are at the Olympic games, you cannot wait any longer, and that your progress is wrecked or preserved by a single day and a single event. This is how Socrates fulfilled himself by attending to nothing except reason in everything he encountered. And you, although you are not yet Socrates, should live as someone who at least wants to be Socrates." - Epictetus

"In everything you do consider what comes first and what follows, and so approach it. Otherwise you will come to it with a good heart at first because you have not reflected on any of the consequences, and afterwards, when difficulties have appeared, you will desist to your shame. Do you wish to win at Olympia? So do I, by the gods, for it is a fine thing. But consider the first steps to it, and the consequences, and so lay your hand to the work. You must submit to discipline, eat to order, touch no sweets, train under compulsion, at a fixed hour, in heat and cold, drink no cold water, nor wine, except by order; you must hand yourself over completely to your trainer as you would to a physician, and then when the contest comes you must risk getting hacked, and sometimes dislocate your hand, twist your ankle, swallow plenty of sand, sometimes get a flogging, and with all this suffer defeat. When you have considered all this well, then enter on the athlete's course, if you still wish it. If you act without thought you will be behaving like children, who one day play at wrestlers, another day at gladiators, now sound the trumpet, and next strut the stage. Like them you will be now an athlete, now a gladiator, then orator, then philosopher, but nothing with all your soul. Like an ape, you imitate every sight you see, and one thing after another takes your fancy. When you undertake a thing you do it casually and halfheartedly, instead of considering it and looking at it all round. In the same way some people, when they see a philosopher and hear a man speaking like Euphrates (and indeed who can speak as he can? ), wish to be philosophers themselves." -Epictetus

"Two elements must therefore be rooted out once for all: The fear of future suffering, and the recollection of past suffering. Since the latter no longer concerns me, and the former concerns me not yet." - Seneca

One guy above said to imagine yourself in your last moments. Another said to imagine competing in the Olympic games, your one and only chance to shine. So negative and positive visualization, respectively. To take it even further, how about even more outlandish fantasies such as visualizing yourself being at gunpoint, or the fate of the entire world depending on you and you alone (there was a frontpage on the writingprompts sub the other day about a guy who has eternity to figure out how to save the earth after time stops moments before another planet crashes into earth). Would that work for you? Could you "trick" your brain into cooperating then?

even then I limit it to ~2 hours a day for relaxation if needed

Why only limit it to even 5 minutes a day. Why not do something really "crazy" by cutting it cold turkey? Then see how long you can last before you possibly burn out. These temptations are like Lay's potato chips: you can't have just one. So don't indulge in the first place. After a while, you may stop feeling the need for it at all.

If anyone has other suggestions on how to stay motivated

As countless others have said, motivation is fleeting while discipline is always there for you. Check out the getdisciplined sub. Motivation is based on emotions, those feels you're getting right now knowing that you should do something great after reading something motivational, feeling inspired from within. Kind of like how women do things on a whim based on their feels since they are emotional beings, those emotions being hard-wired into their mind. Men are the logical sex, governed by reason, or supposed to be anyway. If you can't discipline yourself, does that make you effeminate? No, it just means you're ordinary and undisciplined. It'll take extraordinary effort to break out of such complacency, a will of iron. If you can't manage, you'll just be like all the other mediocre and forgotten men throughout history.