I agree with everyone that says air consumption will improve with practice.
In the meantime, here are some things to try that will help alleviate some of the guilt you are feeling about ending your buddy's dive early.
1. When on the surface, manually inflate your BC (yes, this isn't much but every little bit helps)
2. Stay a little higher than your buddy until there is something good to see. It is easy to stay in close contact with your buddy and still be 5-10 feet higher than them.
3. I know some are saying this isn't a great suggestion for a new diver, but a small stage bottle shouldn't overwhelm you. Maybe think about carrying a 40cf stage bottle, especially to resorts where you probably won't have the option of getting a bigger tank. You will need to take the valve off to fly with it, but most resorts will be happy to give it an inspection, put the valve on, and fill it for you.
4. If your buddy is comfortable with this, breathe off of there alternate air source for a few minutes while descending. If you have 30 minutes of air and your buddy has 60 minutes of air, if you breathe your buddies are for 10 minutes, then you both can have a 40 minute dive and your buddy will still have 10 minutes of air left. Keep in mind that this example is overly simplified and assumes that your emergency reserve is not included in the 30 minutes -- It only serves to show the math.