When is it time to say "enough"?

I am at work right now. I am going on nearly 8 hours with 4 more to go. I am exhausted. I can barely keep my eyes open and I am so tired I am dizzy.
I gave up going to physical therapy. It was recommended by my rheumy because he thought it would help with range of motion in my joints. I went for a month twice a week. The only thing it did was make me even more exhausted and my muscles and joints hurt so bad I did not want to move.
Why am I being so stubborn? Why am I pushing myself like this? Why am I so affraid to go on disability? I know the answer...I am afraid people will look down on me and tell me there is nothing wrong with me and that I am just using the system. After all, I have heard that before!!!
People keep telling me how strong I am....but I am not. I keep telling myself~ "just one step at a time..one day at a time". But I feel like if I take one more step today I am going to fall over.
I am so tired that I can't stand myself right now. If I ask to go home, it leaves the others short-staffed. I sound like that old engine from that kids story book....repeat to myself "I think I can, I think I can" and maybe, just maybe, I will make it through the rest of my shift.