This is a question that we deal with on an every day basis in a commercial gym setting. You can't do any more, or you won't do any more?
Many of our clients are not athletes, nor were they ever athletes. They found fitness and training for a reason, but often times those reasons don't involve wanting to really push their limits. People want to get healthier and fitter, but many don't understand what they are truly capable of. This is where the job of a Personal Trainer really differs from that of a Strength Coach. My clients have no idea what they can do, and I need to teach them that they are capable of so much more. A strength coach's clients (athletes) know that they are capable of more, and need a coach to help bring that out of them.
"I can't" is one of my least favorite things to hear in the gym, especially before you even try something. How do you fucking know what you can't do? Try something hard once in a while, I bet you'll surprise yourself with what you can do. "I can't" means that you're giving up before you even try. It means that you're just going to roll over and die without finding out what you're really made of, it means that you're scared of failing. People are so scared of trying hard and failing at something; they are scared to find out what their weaknesses are. I embrace my weaknesses, that's the only way to know how to make yourself better.
What you really mean when you say "I can't" is "I won't". I won't attempt this lift. I won't keep sprinting. I won't do another set. I won't stop eating sugar covered shit all day. These are self imposed limitations set by someone who has never experienced what they are really capable of doing. One of my favorite sayings is "whether you think you can, or think you can't, you're right". If you tell yourself that you're going to be able to do this last set of push-ups, you're going to increase your chances of success exponentially. If you sit down and say "I can't do it", you already lost.
Most of my clients have never experienced real pain caused by exercise. The kind of pain you have to fight through on the first day of football or basketball practice. Not the pain of an injury, but the pain of exhaustion to the point of not being able to see straight. Lungs burning, legs shaking, world spinning; that kind of pain. The kind of pain you have to mentally fight through. I learned to deal with it because of sports when I was younger. I was always an athlete, but I was never the best. I had to fight tooth and nail to make every team that I tried out for. I made those teams based on hustle. I wasn't going to shoot my way onto the high school basketball team, so I made damn sure that I worked harder than anyone else. Another set of suicides? I'd be the first one on the line. Loose ball drills? Time to leave some skin on the court. Being mentally tough is what kept me on the team.
Getting a little winded or some muscle burn is not pain, that's a little discomfort. That is something you can (should) work through. Don't get tired, get tougher. The human mind is the limiting factor in your potential in the weight room. You are capable of so much more, if you're willing to work hard enough to achieve.
Don't be afraid to work hard, go lift something heavy; find out what you're really made of.