It’s really easy. Trust me on this. I’ve had the kind of week which really illustrates how easy it is to stress someone to the level where they’re not thinking any more, just reacting and trying to hold together until things settle. You don’t even need to do anything drastic.
In my case, a combination of factors meant that I ran out of the medication I need to stay awake (Narcolepsy, for what it’s worth, sucks). That turned the last week into an exercise of watching the mail in a near-obsessive fashion when I wasn’t just trying to stay awake. The world could have ended and I probably wouldn’t have noticed.
Now, what does your character need? What would drop your character into survival mode where the only thing that they can focus on is making it past the next crisis? Once you’ve figured out what it is, do it.
You can play it for laughs, like Pratchett did with the caffeine-obsessed vampire in Monstrous Regiment. Or you can be serious. Either way, you’ve got a character who’s not at their best, and who’s desperate for that one thing they know is going to fix it.
That character is going to do everything in their power to try to get what they need, which is where the author comes in – stressing the character even more by making sure the timing isn’t quite right or things just don’t work and they don’t get what they need. Or – preferably – the character’s impaired state causes them to do things wrong which winds up taking them further from their goal instead of winning them what they’re after.
Eventually, of course, your character either gets what they’re chasing (sweet, sweet caffeine) or they discover they can live without it (oh, look! Character growth).
Or in my case, the medication finally arrives in the mail.