We were practicing the "repulse monkey" sequence (of the short yang form, which I practice) and I was constantly frustrated. Turn your waist, but don't move your head offline from your navel. Bring your hand up back behind you, but don't be stiff; let it swoop up gracefully. At the same time, turn your left palm down. Then, as you return with your right hand, stop it close to your head, meanwhile turning your left palm up in time. Then step back with your left foot. Oh, and don't forget to step with your heel slightly out, so it ends up being straight. Then push your right hand forward and bring your left hand back and down in fluid time. Finally, lock your hips in square, forward, which draws your slightly everted right foot in parallel with your straight rear left foot. Also, make sure your right hand is not too high or low and it not bent too much in to the body, but not extended too far from it either. And then do the same thing, basically twice more, once to the left and then to the right.
And that's just one of about thirty sequences in just the SHORT Yang form.
Shoot me with a harpoon! If I didn't forget to turn my palm down, I forgot to hold my head in line. If I didn't forget to turn my palm up, I forgot to stop at my head. If I didn't forget to stop at my head, I forgot to step back with my heel out. And on and on and on.
Add to all this extremely subtle coordination the need to keep your weight shifted on the correct leg, knees bent, etc. Try holding a repulse monkey for thirty seconds, and then try drilling the steps for it in such a bent-knee position. It's unrelenting, solid isometric work.
"Taiji is the hardest thing I've ever done."
Keep in mind I did rowing for six years in middle- and high-school. And, I'm sure, at the time, I said, "Rowing is the hardest thing I've ever done." And, on balance, it really probably was. Once I step outside my maladroit frustration at practice and look at things "objectively," I realize rowing, with its amazing combination of sheer physical exertion and crucially fine technique, is the most difficult thing I've ever done.
but even so, taiji is the most difficult thing I've ever done in one different key respect: basically, doing it well is all no my own shoulders. In a rowing shell, your decision to "relax" or move poorly is felt immediately by everyone else. The baot suffers if you suck, or if you choose not to apply your training. But in taiji, except for the coach watching youa t times, and then perhaps stepping in to correctyou, doing each move well depends on you alone, as you practice "one on one" with your own body. It's not mentally draining, but it is also not something you can just "fall into," at least as a benginner.
Taiji is hard not primarily because of its isometric demands, or because of its demands for sturdy balance, but because it is so easy to feel, at a moment's notice, just how bad at it you are. It is very Daoist, in this repstct, because it is very humbling.
How well do you know your body? How well can you control your body, whether in motion or in a balanced poise? Taiji will show you. And so far, it's shown me that I am quite out of touch with my body's "gyroscopic" energy. But I shall persist! Incidentally, I've decided to switch to judo once a week and taiji thrice a week, with possible long classes on some Saturdays. I have thirteen more months to study taiji in its heartland. God be with me.
 Gyroscopes work, if I'm not mistaken, on the principle of lateral circular momentum (or whatever a real physicist might call it). As a wheel spins, it actually sends energy out to the side. Thus, circular motion creates a lateral vector that keeps a spinning wheel in balance, literally gyrating between its "competing" lateral spin and its median energy. In your body, though there are no real gyroscopes, there is tendency for muscles to divert away from resistance. So, for example, when you balance back on one knee with the other leg planted forward, your rear leg muscles tend to drift inward or outward to reduce the median "spin" being applied to your leg. The muscular spin is created by the tension between gravity and your leg's resistance. I love kinesiology.