We know what we are, but know not what we may be.

Your current self is knowable. It has a history, habits, a set of things you have already done, and you can describe it fairly well. What you might become has no such evidence behind it, which is why it tends to get underestimated. People mistake the limits of their record for the limits of their capacity. The unknown part of you is not empty; it is simply unwritten.

When you catch yourself saying you are not the kind of person who does something, treat that as a description of the past rather than a verdict. Try one small thing that contradicts it.