Breathing to conserve air is like pushing a rope. It's trying to do things the wrong way around.
Learn to be quiescent in the water, able to effortlessly hover neutrally buoyant in the water without any extraneous flapping, and learn to slow down. Minimizing motion reduces the amount of calories you burn, reducing your CO2 generation, reducing your gas consumption. Learn to be relaxed and calm underwater without worrying about things (like your breathing). Reducing your stress level will also reduce your gas consumption. Wear sufficient thermal protection to be warm (but not overheating). By losing less heat to the water, your body will not need to burn as many calories just to try to keep warm, and with fewer calories used, you'll have less CO2 generated to breathe out.
Trying to reduce your gas consumption by concentrating on your breathing does nothing for your physical exertion level or heat loss. Unless it *vastly* reduces your stress level (and it may increase it, if you're really trying to breathe "correctly"), you won't be generating any less CO2. If you're breathing less and generating the same amount of CO2, it naturally follows that you are retaining more CO2 by your efforts.
Elevating your CO2 level by trying to force your breathing makes you more susceptible to oxygen toxicity, it contributes (perhaps strongly) to narcosis, and it's likely to give you some *royal* headaches. It's just a bad idea. Far better is to concentrate on the things that generate CO2, as when you reduce those (slow down, fix your buoyancy and *especially* your trim, wear the wetsuit) your body will naturally breathe less and your dives can relax into longer times without any buggy whips or spurs necessary.
Breathing is a sign to be read, not a skill to be learned.