The first tap will always be the hardest since it's the initial release of CO2 and pressure from the capsule. If you find pushing the button difficult, hold your bottle at the bottom or place it on a flat surface. Use your thumb or your first two fingers to press down on the button on the Aer1.
Storing your capsules in the fridge is a great idea to help with this. By keeping your capsules cool, you'll lower the internal pressure, making the Aer1 button easier to press.
Over time, each tap will get a little easier.