If worn for a long period, earpieces tend to cause ear discomfort. There are a few ways you can fix ear fatigue.
1. Take breaks. Take the earpiece out of your ear when you don't need to have it in. Even if it's only for a few minutes at a time, it will help your ear breathe.
2. Change the ear tip. Typically, an acoustic tube comes with a standard mushroom ear tip. These are known to fall out of ears easily and be uncomfortable.
One alternative is a semi-custom open ear insert. These fit right on your acoustic tube and offer a flexible breathable fit and come in 3 sizes (small, medium, large) making them customizable.