There's no such thing this as can't... You either will or you won't.
Overweight but active my whole life, it took a heartattack at the age of 27 to wake me up and take my life back. I am a former collegiate football player who sat back and watched my weight balloon to well over 300 pounds, how much over 300? That I don't know. The scales reached their max point at 300. In a matter of 9 months, I lost over 140 pounds through exercise and nutrition. Pushing my limits, I, both physically and mentally, bottomed out and fell into a borderline anorexic state. At my heaviest I was 300+ pounds and at my lightest, 162 pounds. I addressed the physical needs but ignored the mental aspect. It took extreme dehydration and mentally burning out to bring the polar opposites, Obesity and Anorexia, into a healthy balance and a comfortable weight of 185 pounds. I've been in the training industry for 11 years, 8 as a strength and conditioning coach and 3 with general population clients. The certifications I hold are through the NSCA with a CSCS distinction as well as USATF Level 1.
In training, I don't allow CAN'T. If I'm told "I can't do ___", I stop whatever we are doing and prove them wrong before continuing on course. As long as can't survives, can will not. Within physically realistic reason, of course.