If you feel the CO2 is hitting you hard, then you've got a high metabolism going on. Why a high metabolism? Various reasons possible, let's name a few key ones, but I believe there are more possible.
1. You're swimming with fast moving body parts.
2. You're swimming while exerting a lot of force.
3. You're swimming with a high blood-flow.
Why would your body do the above?
1. because you direct it to
2. because you're not rested.
3. because you're ill.
4. because you are not trained to reach lower metabolisms.
5. because you're young, it is natural.
6. because you're cold.
From the video's I've seen of you doing dynamic I would say you move 2 to 3 times too fast. My guess of why, I believe it may be because you want get to a certain distance before you feel that CO2 hit coming. I suggest to try a session of swimming just to the point of when the CO2 hit comes, and try to learn to swim to this feeling in the slowest way, using very little force, slow movements, fluid and efficient technique. Leave the numbers aside, and focus on the above and enjoy a session of good feelings because you're not pushing beyond you CO2 hit, you actually stay under it.
As for CNF,
It is the most pure, and the most difficult discipline.
Difficult because one need to do al lot, and with precision!
For instance, miss an equalisation? - you have to stop, equalise and get going again, and getting going without fins is more demanding.
Equipment makes a lot difference in CNF, for instance having a noseclip allows you constantly equalise without needing it to time with your arm stroke movements. A flexible smooth thin suit allows for easier limp movements and less drag or more glide, which means you can slow down on the swimming pace.
If I have only a inflexible suit, I would adapt my tactics. I would dive FRC, or less then full lungs in such way that I would have neutral buoyancy at 5-7m. I would pre-equalise and have a very good duck dive (ass and feet high) so my feet are under water before I start swimming. When I start swimming I equalise when I bring my arms up for the next arm stroke.
It's a cool challenge to discover and align and optimise all these technical parts into 1 easy, efficient, consistent, automated routine.
When I reach neutral buoyancy I can add 1 or 2 more strokes and start gliding.. ooh that delicious gliding
If my tubes are warm, open and flexible, and my technique is ok, I can do hands free with my Sphera mask on. This is a party in itself, it's gravity powered flying. If my ears are not equalising with ease, I hold my nose gently with my right hand (where also my computer is), and my arm in line of the water flow on my chest, for good streamlining. Now the only thing I need to do is the rhythmicly equalise and relax and enjoy the hug of building up water pressure. If I cannot keep up and get behind on equalising, I hold the line with my left hand and sort the equalisation out before going any deeper. Now a nice tip on getting back into deep diving fast after the pool season: at the turn, take a moment or two to just hang and relax al your muscles to get used to relaxing under pressure. I found that doing longer bottom times at a 'shallow' depth are much more productive then touch and go sprints to increasing depths. For beginners doing a relaxing sit or lay down on the bottom at or just below neutral buoyancy is great to learn to relax under pressure, and becoming comfortable at depth. If you can reach that neutral depth in a relaxed comfortable way, you can understand that going deeper is just getting easier because there is no more coordination with swimming. The one thing that is then becoming difficult, is to know where the half way point is, when to turn in order to get to the surface without BO. I've learned here that, just like with dynamics, sprinting and or fast action stands in the way of feeling.
Ok, I feel I'm writing a book here, there is probably already too much information for beginners to chew on.
In summery:
Analyse your CNF technique, and all it's parts.
Work on getting and keeping your metabolism low.
Allow your body to get relaxed and used to depth.
Improve your gear.
Do not push limits all the time, otherwise you'll teach your mind that freediving is stress and pain, which will not last as a hobby.
Love, water and Courage,
Kars