Here's a little metaphor I often consider...
We can consider techniques on many, many levels (the actual definition of "technique" is hard to make precise). One way that makes sense to me is to liken building my technique to building a house or other building.
The very first place is the foundation. Without a strong foundation, even house will fall in the slightest bad weather. This means that we train the kihon dosa until our legs and hips are strong, and out stance solid. This dachi is the foundation for all technique and under any circumstances, your feet should always return to kamae in order to maintain balance.
Usually, we find that when doing Jiyuwaza, once we start to lose the proper footwork of 180 degree turns and irimi of the hips, the rest of the techniques collapse. We begin to tilt or lean forward, and lose all the power. The loss of shisei is a direct result of the loss of dachi and no technique can manifest when that happens.
So it is important to consider how to make that foundation strong, and to always spend time in practice on the kihon dosa to give muscle memory to the movement of the feet and hips. Once this happens strongly and without the body rising up (actually the feeling should be one of sinking, not rising or floating) then we can begin to let the techniques manifest more fully and we will not get tired or lose kamae so easily.
Later, with a strong foundation, we can build a house of any size or shape. In particular, we can think of "zoning", a concept often used in JKD training, to divide angles of attack into different zones or quadrants. Western fencing also does this (and may be where Bruce Lee got it). Responding to zones is a lot easier than responding to specific attacks. We can also consider movement inside/outside of uke, high/medium/low zones for response, and other tactical elements, none of which work if the foundation of mobility is not strong.
Every martial art fundamentally needs the foundation of mobility in order to be effective, and Yoshinkan is no different. Please take time to work on this. It makes a big, big difference.