You've never seen the cliff
Take a moment to think about every time that you've been pushed to the "breaking point." Just what is the breaking point? Shouldn't the breaking point be where you absolutely can't take anymore and you either die, pass out, or shut down? You would think so, because that's how I would define it. However, with the way our minds are constructed, our brains have ways to keep us from even getting close to that point unless it is absolutely necessary for survival.
I'm posting this in hopes that you could realize just how deep your capabilities extend past your anticipation. Yesterday at track practice, I was pushed to the point where I literally thought my body was going to shut down and stop functioning. This was after the second set. There was actually a third set that I didn't know about. When I became aware of the next set, my brain went through a whole series of "this is rediculus's and he must be crazy's." But there was something different about my mood that day. I thought I would do a little experiment with my body. At this point, since completing the workout seemed hopeless, I would keep going until my body would tell me to stop. The going got rough, but my body would not give up no matter how much pain I was suffering. I was suffering to the point of insanity... no lie. But I kept going because I wanted to see just how far away that wall is. I want to know where that metaphorical wall is; the breaking point. Wouldn't you know it, I never actually did hit the wall. I was suffering to the point of insanity and yet I did not reach the breaking point. I may have been pretty close, but I probably could have completed another set or possibly two before my body began to shut down.
Even though all of this exertion made me insane yesterday, pushing myself to the same level at a later date would not have the same psychological effect because it has happened before and trust that the amount of pain I was suffering was ok. Therefore, even though it is extremely difficult, you must realize the large buffer between your pain threshold and your actual breaking point. You would be able to cut off your whole arm and survive. Think about that one for a second.
In order to test my pain threshold, right after writing this post, I stabbed myself with a sewing pin by slowly increasing the pressure until I could get it to bleed. It was very difficult and took a few tries, but eventually I just took a deep breath and really shoved it into my finger. You have to trust yourself and just do what your brain tells you that you are not capable of. You will surprise yourself, and your willpower will only increase. The exponential theory in a nutshell... do what must be done immediately and the rest will follow.