Whilst I have enough knowledge and experience for providing some effective hints, I prefer not to pass you these info.
Breath holding is a very dangerous practice. And free diving at depth is even more dangerous.
When I was 20 I did train for free diving, and during this training I incurred in shallow depth blackout twice.
In the swimming pool and under vigilant eye of the trainer, who rescued and revived me promptly.
One of the two episodes was a true near-death experience.
This convinced me to give up entirely free diving, and focusing on scuba diving only, which is far less dangerous (despite I was doing deco dives in air down 50+ meters).
So, if you still want to practice breath holding, I warmly suggest that you train only under the guidance and supervision of a qualified instructor, capable also of rescue and resuscitation.
A good free diving instructor will also provide you these hints on how extending your breath-holding time.