I don't think I can do it

I don't think I have what it takes.  At my job, I wear out.  Before 5:00 rolls around, I'm already done.  I'm so tired.  I just can't work any more.

Is it that I just don't have the strength to work full-time? Or is it something about my job in particular that wears me out? It could either be lack of interest in the tasks at hand, or something about the physical environment in my office.  My job is not physically strenuous, and my office seems physically comfortable, but I know that sometimes people are sensitive to things like toxins in the air.

They say you can do anything if you work hard, but I just don't think I have the physical ability to do anything.  I try to work hard but my body does not cooperate.