One of the mistakes I see beginners constantly make is putting themselves off-balance by either leaning backward when the stand or sticking the hip out of alignment when they pull or push.
In the first photo, you see the white line showing how the line of the hip extends out much farther than the edge of the foot. When we aren't paying attention, we have a tendency to let this happen.
If you sink more into the kua and bring the hip into alignment, the edge of the hip is more in line with the edge of the foot (photo 2). In a posture such as the two-hand push, where you are on the toe of the left foot, it takes a lot more leg strength to pull off this proper alignment. If you hold this stance for any length of time, and your hip is aligned, you're leg will become quickly fatigued.
When you attend a workshop or a private lesson from someone like Grandmaster Chen Xiaowang, he'll ask if you want soup or pizza. If you say soup, he'll put you in an easier stance that your muscles can tolerate a little better. If you say pizza, he'll put you in a proper stance, and your muscles will quiver and burn and let you know very quickly that they're feeling it.
Good Tai Chi is painful. It takes tremendous leg strength to do it right. No pain, no gain.