@TecDiver - Do a series of 6-8 breath holds. Fully relax between the holds for a period at least long as the holds were.
The first one should just be a warmup - just hold long enough to be uncomfortable for 15-30 seconds. Maybe the second one a warm up also - maybe a bit longer.
But for the remaining holds do this: push yourself way into the struggle; when you feel you just can't go any longer say to yourself "surely I can hold for 5 more seconds" - then do just that. Hey, your still OK. And now "surely I can hold for 4 more seconds". Just do it! Then "3" more - and then 2 more and then 1 more. DO IT DUDE! You just went 15 seconds past your "give up point". Count those seconds in your head or look at a clock. 5-4-3-2-1... 4-3-2-1...3-2-1...2-1...1
Think about this, breath holding does not actually hurt, it's just an urge which you need to become familiar with and learn to love. Would you rather someone hit you on the thumb with a hammer or would you prefer to hold your breath a bit longer. For me, I'd rather struggle through a breath hold than have my thumb smashed. Ever been in a car and had to pee but there was nowhere to stop? It can be a strong urge, but somehow you force yourself to not pee your pants. It is willpower.
So how do you over come the urge? Practice. After weeks/months of practice you will becomes very familiar with the sensations in your body and you will know when a breath hold is going to be good or bad. You get a feel for just how much air is in your lungs - maybe you inhaled too much and feel too "tight", or maybe you could have inhaled more. After a lot of practice you will become very in tune with the feelings in your body.
And part of being in tune is the feeling of the urge to breath. Again, it is an urge, not pain. Use your mind to "transform" this urge into a pleasant feeling - mid-over-matter. Just go with it and imagine that it feels good. What is the best physical feeling you have ever experienced? Use you imagination here. Well, as you progress through your breath hold and start to feel the urge to breath say to yourself "this feels awesome, I'm alive, I can feel the power in my awesome huge lungs, I'm having an o----m , I want to this to last forever !!!" Then make it last forever. After a lot of practice you become familiar with the urge and what it feels like to push into it. It can be dealt with and actually enjoyed!
An experienced breath holder can force themselves to the point of blackout. I think it is instructive for all freedivers to at least once try to push themselves to the point of blackout - it serves 2 purposes. One, it teaches you that yes indeed you can overcome the urge to breath and hold your breath a long time - and two, it introduces you to the feeling of what a blackout feels like - some get tunnel vision, ringing in ears, dizzy, bowels start to squirm. Of course this is not healthy, doing this once should be enough. When you black out you are at the point of starving your brain of oxygen. And of course NEVER do this in the water.
Having said all of the above, I also agree with with John Griffith - you can over train. If a breath hold session seems lifeless or you don't feel into it - just stop, rest for a few days.
