OK. This is a simple fix. The problem is caused by not being able to train the part of the dynamic near the end of the swim. (Obviously, every dynamic we do trains the start and middle of an effort, but rarely do we get to train the hardest part, because we rarely do maximum efforts) The fix is to train the end of the dynamic and get used to the transitions between the easy, transition and competive stages of the swim. Do not try this exercise alone.
After warming up your breathold. You are ready.
1. Take ten really good long slow breaths and do a 25m dynamic. (all the dynamics must be gentle and not racing)
2. Then take nine breaths. The time it takes to do these breaths is your rest period. Then do another 25m.
3. Then take eight breaths, 7,6,5,4,3 etc. Each followed by 25m.
You will recognise how the easy stage becomes a transition into a harder stage. You are now training that transition. Somewhere around 5 breaths, you will notice that the rest period is much reduced and the 25m is almost all a struggle. This is similar to your 50 to 75m section of the dynamic. You will then do a 4 breath rest,then a 3 breath rest.
You will now have completed the last three 25m swims as if they were all the last length of a maximum effort. You will have experienced all of the mental factors that limit your performance.
Then, after you have done the 3 breath rest, you go back up to 4 breaths, then 5...back to 10. You will be very anxious re-doing the 4 and 5 on the back of the struggle of the 3. Remember, you are training the anxiety and recognising the feelings.
During your breathing/rests, do not speak etc, even coughing will mess things up.
Of course, many trained freedivers might be able to re[peat the breath/rests down to one breath and back up to ten.
For me, I never tried beyond going down to 3 (I trained alone and didnt ever risk it). I doubted I could ever do it on 2 anyway.
But, I was always aware of my body after repeating the exercise a few times over the period of a couple of weeks.
I only ever did 3 maximum efforts before I couldnt compete anymore. 131m, 139m 156m. Pretty ordinary today, but mine were 8 years ago. The exercise I have mentioned above, was the best I ever figured out. My static PB was 4:15. I reckon I must have been doing something right as many 6 minute plus statics cant get close to that 150m turn even today.
So its simple. Get used to the feelings at the end of the dynamic, because its those feelings that cause many people to stop, when they could otherwise easily add another 25m if they recognised them.