If your last breath is screwing up your holds, try doing only the normal ventilation.
The key is to ventilate slow and as little energy as possible with it.
Best is to lay down on your back, rest your hands on your belly, and breath in and out by using ONLY your belly, leaving your chest flat. The begining tempo should be about 5 sec in, 5 sec out. Breath through your nose only and try to follow the air as it finds the way in down to your lungs. Try to make the airstream constant, with a slow start and with a slow end. Try not to go to the very end positions of your belly breath, this is where you need (too) much effort to fill or empty the last 10-20%. A proper belly breath is very important, you can practice it all day if you like. It's the way to breath for humans, the cheast breath uses to much energy, and isn't -in general- benificial for freediving. It's only good for panic, fleeing, running situations, people with only chestbreathing may be prone to much stress.
Ok, next time you make your biology lessons more intresting, try to a small schedule. If your max is 1:20, take half of the time, 40 seconds as the holdtime.
Now try to complete this schedule:
2' breathup, only belly...
40" hold,
1'45" breathup
40" hold,
1'30" breathup
40" hold
1'15" breathup
40" hold
1'00" breathup
40" hold
0'45" breathup
40" hold
30" breathup
40" hold
15" breathup
40" hold.
This should a pretty tough challenge, try not to get noticed
You may find it handy to start the breath out in the last 5 seconds, slowly that is. Try to maintain a slow breathing tempo, with little effort.
Please train safe, and don't do max breathhold all the time, they are very tiresome, and you may exaust your mental will as wel. It's better to alter your days like: day 1: stanima, 2: flexibility, 3: breathhold, 4: rest. 5 swimming technique etc. Don't practice breathhold in water if you don't have a very capeble buddy which knows, and does help, if something goes wrong. I wouldn't want to find out such a accident. Breathhold while driving something isn't also very smart. See if you can follow a proper freediving course, good luck!
Kars,