Accept the fate the nature has burdened you with, and learn to live with your disability!
Youth, such a hopeful time. Most of the plumbing and hardware still working as intended, and relative ease of pulling oneself out of whatever auto-destructive behaviour tickles your fancy, breeds the belief in one's infinite potential. In a linear relationship between work put into self-improvement, and results.
Alas, young cub, that shall never be. No matter the effort, you'll never overcome the limitations so generously bestowed upon you by the random chance upon conception and early years of development.
Oh, you may work out, you might study hard. And yet, you'll never be a Batman nor a Bruce Lee.
It's an illusion, you see. For, soon you hit the ceiling of your potential. You plateau with the results of your hard work. You encounter an unavoidable obstacle to your naive dreams of glory.
And as the payoff of the ever-increasing amount of effort diminishes, you find less and less motivation in you to continue, spoiled by the early successes.
So you can't stand for prolonged periods? It's the nature's way of saying "I've got your balls and I see no reason not to crush them", where "your balls" is a metaphor for the fantasies you'd had about your future self.
You may think of it as a bizzare dickishness of life, but the truth is you've just experienced the stubborn denial of reality that defines the human condition. And indeed, will you not continue in refusing to remove the blindfolds from thine eyes? Will you not travel to a medicine man and plead for drugs to fool your body into believing that yes it can, after all, stand straight without passing out?
Ah, the meds, the sweet pills of oblivion. Aren't they the most devious gateway drug?
Why stop at whatever presription you get for your present problem? Why not fight other limitations by artificially altering your body's chemical processes? Can't develop any more muscle? Get them steroids, boy! Can't focus anymore? Metamphetamines are here to give you a shaky hand and a yellow-toothed smile. You can't dream anymore? Venture to the land of LSD.
Doomed, that's what you are. You can either foolishly seek to conquer your limitations and end up a junkie wreck of a man, or learn to live with the newly found realisation that the quiet desperation is all that you'll ever get from life.
And then, once you're old and decrepit and crippled by the inexorable march of entropy, you'll think to yourself: "Why?"
"Why did I not go to the doctor? The fuck was wrong with me to seek health advice on the internets? How bonkers must I had been to follow some hairy, overweight, basement-dwelling ogre's* suggestions? I have wasted my goddamn life! Nurse! Nurse! Let me grab your left boob! I want feel alive for once, nurse!"
*not factually accurate