Can and Did
Although Longfellow's pronouncement has superficial plausibility, it's merely an example of polar logic. One pole is what you feel capable of doing and the other is what you have already done. The judgment reduces to can and did. You judge yourself based on "can" and others judge you based on "did," according to Longfellow. The reality is that such judgments rarely reduce to either can or did, for you or for others who judge you.
Look first at "can." If this is a judgment you make about yourself, is it reasonable to make it without considering "did?" Relying exclusively on what you think you can do, without considering what you have done, places no value on prior experience. It also acknowledges an inability to learn. Alternatively, if you consider did to the exclusion of can, your behavior is simply repetitive; and you will need to take Albert Einstein's observation to heart, "Insanity: doing the same thing over and over again and expecting different results."
Look next at "did." If others are expecting change, improvement, innovation, or new approaches and strategies, you aren't the person they need. They can only expect you to do again what you did before. Unless "can" is considered, nothing new or different ever happens. The conclusion is that can and did aren't separable. They are the head and tail of the coin of progress.
How then should one approach success? What is the best strategy for blending did and can? Arthur Schopenhauer pointed out, "a man can do as he will, but not will as he will." The message is that you can't simply "will" things to happen. You have a wide range of options for doing but no magical powers. Alexander Graham Bell said, "The most successful men in the end are those whose success is the result of steady accretion