Take a freediving course, you'll learn so much.
Did you think of staying hydrated?
The key to breath-holding is relaxation.
Just go to a warm pool, have a buddy and practice static. Your buddy can see where you body is tense. He can also move parts of your body so you can yourself notice these parts are tense. When you manage to relax, the waves will go through your body, and you are just floating like a towel on the water. Now you can do the same while breathing through a snorkel for your preparation with spearfishing. You'll see the more relaxed you start the dive, the less O2 you consume, the less CO2 you produce, the longer you can stay down with greater comfort, less tension, less heart-rate.
Minimising your movement in the water is also key. Behave like a fish, always move SLOW. Now as to breathing, practice this on land and in water, breath from the belly only, very gently, 5 seconds in, pause 2, 5 out, pause 2 ... This should become a natural and fluid like movement. Your mind and body will relax when you breath slow from the belly.
During the dive always move slow, and minimise movement. Also avoid stretching or straining your neck or straining any other parts. As a result you'll keep your chin tucked in when duck diving, improving streamlining and relaxation.
Coming up, swim economically, smaller amplitude kicks, good hydrodynamics, nice rhythm, relaxing your diaphragm and body as much as you can. At the surface take 3 hook breaths and exhale slowly, put snorkel back in and relax for the next dive.
Make sure you got correct weighting for your target depth.
Do not eat heaps of slow digestion heavy foods before. Light stuff, easy to digest stuff a FEW HOURS BEFORE diving really helps. Drink lots and have some bottles on your float so you remain hydrated.
As for practice, dive with you buddy and just have long fun dives focussing on the points I gave you, learning how to relax before, during and after the dive. Do not wear a watch because it's a distraction but have your buddy spot you instead. Focus only on enjoyment and relaxation.
Relax, have fun and eat some fresh fish