When my dad was fighting cancer, we would occasionally go for drives around our small town, so dad could get out and experience the outdoors. Some days, we would go to the local park at the north side of town. They have a short, paved walking trail around it, that was just short enough for dad to get some exercise and for him not to feel winded. Near the end of the walkway, where it curved along back to where we parked, there was a short incline. Really, I wouldn't even call it an incline. A slight uphill grade, if that. But dad was so weak, that this was truly a struggle for him. As he would approach this "hill," he would say, "Here comes the hard part." He would literally take somewhat of a running start to get up the slope. My dad, who was an accomplished athlete and coach, could now barely walk up this sidewalk. I remember as the weeks went on and we would visit this park, he would eventually need a walking stick to navigate up the slope. He could do anything, but suddenly, he couldn't.
When I get to "that place" of hitting the wall in my workout, I go back to that sidewalk; watching my dad struggle to walk, but still giving it everything he had. He got up the hill every time he approached it, but you could physically see the effort and struggle it took him. Right now, there are people like dad, fighting up that hill. They are fighting for their families, their health, and their lives. Surely, I can lift 1 more weight, do 1 more burpee, or run 1 more step.
Where does your reserve come from?