Odds of Hanging On
A courier is on the road...a lot. They can spend three weeks out of each month transporting nuclear materials around (source). If you're someone who likes eating healthy, home-cooked meals and sleeping in a real bed every night, the job is going to drive you bananas.
Dealing with the around-the-clock travel can also be a real bummer for people with families back home, and the hours can be demanding for anyone. Being on the road almost five days a week can be physically and emotionally hard for anyone, so you really need a love for driving.
Although the hours are pretty rough, couriers are responsible for transporting some of the most dangerous stuff out there. People that stay in this job are in it for the love of security and ensuring the safety of...well, everybody. If you can appreciate that aspect of the job, you should be able to do okay.
You've also got to be pretty comfortable, conscience-wise, with driving around nuclear weapons and weapons materials. Couriers aren't always driving around nukes, but if your moral compass points you far, far away from nuclear weapons, you might want to steer clear of this job.