It doesn't need to be this really complex thing. Having a mental breakdown is like the last straw. You just can't take it anymore. I call it "The Unstable Element," which is where you appear normal, but something in you has broken from the struggles of life and it has made you quite unstable. You never know what you're going to do next and even though you think you're okay.. all suddenly.. the store is all out of rootbeer.

"THERE IS NO MEANING TO LIFE. I'M GOING TO DIE. I SHOULD HAVE DIED. WHY DID I SURVIVE?!" A point where you no longer have faith in humanity, your country, your parents, your religion, reality itself, and/or yourself. It's the whole "Screw it," moment where you have no idea why you are. You are beyond broken. There is nothing left. The old version of you is dead. I may or may not know this from personal experience.

My point is that when you are that close to snapping anything can be the final thing. It's like being over the ledge. You're already too far to cling your way back. All you're doing is waiting for gravity to kick in. I got that part from someone I know.