Guard playing/Guard Passing.
So, fundamentally, Brazilian Jiu Jitsu is all about the creation and destruction of structures.
We create these structures with our bodies, connect them with an opponent, and seek to destroy them to create advantageous positions.
As an example, when we play Guard, we create structures to create space and tension between our opponent and ourselves. This creates pathways of potential energy, that we can then release to channel that energy into attacks, be it a sweep or a submission.
Some examples of structures we create in the guard are: Closed, Half, X, De La Riva, Reverse De La Riva, and so on. Every structure of the guard is designed to negate the strengths of the Guard Passers structure and create opportunities for attacks.
Each type of Guard has a specific structure that is uniquely appropriate for a momentary orientation, angle and combination of limbs and positions. If you are having difficulty maintaining the structure of a certain guard in a certain position, it is likely that another structure (guard-type) would fit that position better (it is also possible you don't understand it well enough and it requires more practice). You need to understand all of the guard positions in order to be able to play "The Guard" well. Trying to play a inappropriate style of guard against a certain passing structure will end in frustration, like trying to fit a square peg into a round hole.
On the flip side of the coin, we have Guard Passing.
To efficiently pass the guard, you first have to create structures to put your body in the strongest possible position to be able to combat the off-balancing power of the Guard.
To move forward, now you must destroy the structures of the Guard. Every possible position of the Guard has it's destructive counter-point (Guard passing position and posture), and whoever attains the strongest, most appropriate structure first is able to collapse the other's structures and gain an advantage.
To give this a training application, if you're having difficulty with a certain style of guard, or a certain guard pass, look to your structure. Is it an appropriate response to the structures that are applying pressure to you? You may really connect with Spider Guard, but what if another guard would serve you better in a certain situation where you might be trying to shoehorn your favourite in? You may always want to make the knee cut pass happen, but against an opponent who plays with their knees close together, you're going to have a tough time finding your opening, and in trying to force what you WANT, you might miss a much easier pass that's already presenting itself.
As you improve your structures and your understanding of which structures to create when, your ability to destroy your opponents structures and impose your will will increase.