Your comfort zone could be described as the depth you are 100% certain you can do. There will be no nerves before the dive, you could do it with a short breathe up or rough conditions and you're equalisation will make the depth 9 out of 10 times.
Your comfort zone will not always increase in proportion to your current depth. For example if you are going deeper on most dive sessions, you may quickly find yourself "out of your depth" so to speak, and although you are physically capable of the dive in terms of equalisation, you will start to have problems with relaxation. This in turn will effect EQ, your O2 consumption and your enjoyment of the dive.
When I meet with other divers and speak about our training and PB's, I am often asked the question "what is stopping you from going deeper?" to which I will answer "my intelligence" - provoking a confused look. There seems to be an attitude in freediving that you should always be on the edge your limits of comfort/EQ/hypoxia. But I believe that being on the edge will do more harm than good in the long run.
If you are on the edge, failure becomes a distinct possibility. Whether the failure is due to equalisation, hypoxia or turning early, you will be programming yourself to fail. With that comes the negative feelings that come with not meeting your expectations, which will be compounded with each failed dive. These negative thoughts will haunt your breathe up and dive leading to serious mental blocks and making each dive harmful to your overall progression.
Progressing in freediving requires an intricate awareness of your body and mind, and a focus on the many subtleties which are necessary to reach your targets. Thoughts of failure/feelings of apprehension are only going to distract you from this state and must be avoided at all costs.
The value of diving within your comfort zone is your mind is free to focus on the task that you give it. The problem with diving outside your comfort zone is it will become impossible to stop negative thoughts from distracting you from the very things you need to focus on in order to prevent failure.
I have increased my depth without frustration because I have always given increasing my comfort zone just as much importance in my training as increasing my PB's. The most simple way to increase your comfort zone is to repeat your PB as many times as it takes until the dive is no longer a big deal to you. How many repetitions it takes will depend on you as an individual and you will have to be honest with yourself about how each dive feels and how the thought of a new PB makes you feel. Diving this way can extend your comfort zone past your current PB making a new PB just as relaxing as any other dive. This is how I approach my own training.
If you do encounter any persistent problems at a specific depth then you have found the limit for your skill level. There is no sense in staying at your limit, this is not a place to learn, this is a place to test what you have learned. The key is to go back to your comfort zone, master the skill which is holding you back, then test what you have learned with a deeper dive.
For example if you are running out of EQ before 60m you should keep doing 55m until you can reach the bottom plate with air left to EQ deeper. Only then try for 60m.
What is the difference between trying for 60m and turning at 55m and setting the line to 55m? The difference is each dive will be a success when the line is set to 55, so not only can you train your EQ but you are now programming yourself to succeed not fail. Getting to the bottom plate is a hugely rewarding experience and a valuable part of your training.
Never underestimate the importance of psychology in freediving. From the beginning you will hear that freediving is mostly psychological, and it only becomes more important as you get deeper and deeper. So be sure that you are always aware of negative feelings associated with any aspects of your training, this is a sign that you are out of your comfort zone and should take a step back.
I hope this is of help to some of you out there. Let me know if you have any experience or thoughts on this topic - or even better if you apply it to your training and see the results!
Take it easy and dive safe.