I agree with Dan that you might want to look in to taking a Free diving Course. It definitely gave me a lot more confidents to dive deeper. If you are unable to take a course there is a good book called Manual Of Freediving by Umberto Pelizzari. This book go's over some freediving fundamentals that will increases your bottom time. It will make you a little more aware of what is happing to your body when holding your breath underwater. In addition it will teach you more efficient ways to breath by using your diaphragm mussels to maximize air consumption. I hope this helps you . Dive Safe!