First off, I want to echo what some other people have said, and praise your instructor for not passing you when he didn't feel you were ready, for whatever reason. Spending some more time in the water with you at the cost of equipment rental (which, if it's wetsuit, tanks and BC, seems pretty standard), is a very nice thing for him to do. Making sure you're comfortable and competent before you're certified is actually a great way to increase the likelihood that you will go on to be an active diver.
As far as dealing with gas consumption, there are three things that contribute heavily to blowing through your air. One is nervousness. Someone who is anxious or stressed has a much more rapid respiratory rate, frequently with a shallow, inefficient pattern, and blows through gas. In fact, more rapid breathing can be a personal clue to stress underwater. I don't know how comfortable you felt in your OW dives, but if you were uneasy, you undoubtedly used more air.
Second is respiratory pattern. You will have been taught in your class to breathe slowly and deeply. The reason for this is that, in each breath, a certain percentage of the gas is used to fill up your mouth, trachea and major bronchi, structures that don't participate in gas exchange. This is called "dead space". If you are taking very small, frequent breaths, then a higher total percentage of the air you move through your body in a minute is filling the dead space. With longer, deeper breaths, the dead space becomes a smaller percentage, and you end up having to breathe less per minute to keep your CO2 level steady. So slow, deep breaths make the most of your gas.
Finally, gas consumption is related to effort. More muscle activity translates into more oxygen need and more CO2 production, and therefore increased breathing. Trying to travel fast underwater uses gas, which is one of the reasons we try not to swim INTO current. One of the most frequent sources of inefficiency underwater is a diver who is not truly neutral. If you are diving with your feet below you, every kick drives you upwards. So that you don't ascend, you keep yourself a little negative. This means a part of the effort of every kick is completely wasted; it doesn't drive you forward, it just keeps you from sinking. The solution is to be able to be horizontal, so that your kick is aimed forward, and you can dive at neutral buoyancy.
Hopefully, this will give you some ideas. Get your weighting close to correct (do a formal weight check at the beginning of your dive). Arrange your weights so that they permit you to assume and keep a horizontal position in the water. Don't rush around. Keep your hands still. Relax, and breathe with a slow, deep, rhythmic pattern. All of these things will reduce your gas consumption.
