The Worst Thing About Being The Guy From "Memento"
There are clearly a lot of reason why it would suck to be this Memento guy, but I have to say the main one that I would feel the worst about is dealing with taking a crap. Let's say you're Memento and you need to take a dump. Most dumps take more than the amount of time Memento can remember so he's probably waking up on the shitter having no idea how far along into the dump he is. He doesn't know if he just started, if he's almost done, if he IS done and should start wiping, or if he already finished wiping and flushed and he was just hanging out reading.
If I was Memento I'd probably end up dying on the toilet because by the time I figured it was time to stand up and finish the day I'd have my memory erased and wake up all over again sitting on the crapper. I'd keep regaining consciousness with a fricken beard and mustache smelling like crap wondering what the hell was going on and then I would die a horrible slow, torturous death that felt like dying a thousand times all on the toilet. THAT is why I would not want to be Jeffrey Memento or whatever the hell his name was.