Fatigue

Your lips, tongue, shoulders, back and all other auxiliary muscle groups should not get tired when playing...

Your LIPS are just a gasket, all they do is create a seal on the face of the harmonica for air to not escape. If your lips are getting tired it's because you're confusing intensity with tension and most likely you're squeezing your lips when bending.

Your TONGUE just needs to be placed in the correct location in your mouth to tune your mouth to the pitch that you desire to achieve for a particular bend... no tension is needed.

Your SHOULDERS, BACK and any other muscles have nothing to do with playing the harmonica, keep those muscles relaxed. Standing erect, keeping your elbows at your sides and relaxing your shoulders will help a lot.

Most of these issues come from trying hard to achieve a particular technique (such as bending) and are often habits left over from the learning process that you no longer need. For example... it's hard to say relax when someone is trying to bend for the first time. But, after time a student develops the skill to just move their tongue to the proper place in their mouth to achieve the bend and no longer needs to activate auxiliary muscle groups... relax... focus on the movement that is needed and get rid of what is not.

“Do nothing which is of no use” Musashi Miyamoto (A Book of Five Rings, Page 49)