@ubaldo45 When airway clearance causes additional fatigue, it is possible you are working too hard.

At first, we think we must clear "every last bit" of mucus from our lungs. Remember, our airways are meant to be bathed in mucus, so we just need to move the excess, Trying to get rid of it all can cause us to produce even more mucus.

And unless we have had it clearly demonstrated, the active cycle of breathing and huff coughing may become coughing too hard - tiring, irritating to the throat, and possibly even making our ribs hurt.

There are numerous helpful videos on how to do better, easier clearance, including demonstrations of active breathing, huff-coughing and other techniques.
Here is a longer video that is pretty comprehensive: