When I hear these words, I see a wall that needs to be brought down. There are mindsets in place that are preventing the possibilities from becoming reality. The more firmly you believe you can't, the more work needs to be done to bring the wall down, but it can be done.
The first step is just recognizing that the wall is even there. The wall is beliefs we hold true about ourselves. We may believe we are bad with math, and therefore can't be good with accounting for our finances. We may believe our fullest potential is topped at middle management and therefore we're maxed out in our career. We may believe our clients can't afford anymore than what they're purchasing from us already, and therefore we don't offer them additional services.
Whatever our beliefs, they are boxing us in. We built the wall ourselves by choosing to believe it and maybe we even had help building the wall, but regardless of how it got there, it's there. Placing blame won't help our cause, we just need to work on tearing it down.
Here's your challenge: when you find yourself in a challenging situation, catch yourself saying or thinking "I can't." Write it down. First discern whether the statement is accurate... is it that you can't, or you won't? You don't have to do anything you don't want to do. But if you want it, imagine what would happen if you could do it. What needs to happen in order for you to make that happen? Ponder it until you can tear the wall down. Make it happen.