I have lousy sinuses. I just need air so I'll use whatever lets me breath easily and fast while on the bike.
As for your endurance, maybe you are just in a plateau. You'll have many of them for various things all the rest of your cycling life. Though perhaps if you are riding every day you might need to change and give yourself a few rest days. I tend to not ride on the weekends, and those are my big rest days. And during the week I might ride 2 - 3 times. Sometimes every day.
15 miles isn't a very long ride, but are you hydrating well? If that takes you a hour, you should be consuming 16 - 24 fl. oz. of water or other fluids. If you weigh before and after your ride you shouldn't be very much lighter after the ride if you were hydrating properly. Some say 2% loss of body weight is okay. The more you go beyond that much loss, the more you might feel tired and fatigued that night or for a day or two after.
As well are you replenishing the carbs you burn up while riding? If you ride at a very hard effort, you'll need to replace the carb's during and after the ride. If you ride at a moderate to low effort, then probably not a issue.