Nervous breakdown (or mental breakdown) is when the tension from stress gets so high that the mind and identity of a person break under the pressure.
How can stress, a thing which is not physical, break a person? Stress is a result of inner conflict: we feel like we have to do things we don't want to, and be someone we don't want to be. We try to delude ourselves and endure (this manifests as stress), but if we find no way to resolve this inner conflict, some part of us has to give up and break down.
What breaks during a nervous breakdown is the identity of a person - it is the way we see ourselves and which determines how we interact with the world around us. The way we think, the way we feel, our behavior, our habits etc. all together form how we see ourselves, and when this perceived identity is torn apart all that are left are pieces.
Breakdown is the ultimate way to end stress - through self-destruction. Thankfully, a nervous breakdown is not the end of the world. We are perfectly capable of rebuilding ourselves any way we choose even if it means confronting our inner demons.
What doesn't kill us makes us stronger. A nervous breakdown forces us to look deeply within and create a better 'self'.