John the Little was eighteen when he left home to seek guidance from a spiritual master in the desert. His training lasted twelve years. John attracted disciples of his own, so many that--legend has it--he had to dig himself a cave to find some solitude. (He is not unlike Jesus, who often retreated to prayer.)
Yet, what is John the Little trying to say here? Self-criticism is the light burden. Self-justification is the heavy burden. We have let go of the light burden and bore the heavy one.
He is saying that we are too easy on ourselves--in a way. We do not examine, question and criticize our inner and outer life. Often, we give ourselves grace without recognizing our failings. However, healthy self-criticism is what we might call self-awareness. We must take stock of our strengths and talents, but also our weaknesses and needs. We must examine who we are and how we are in order to grow. Self-criticism is a necessary step towards self-knowledge. It ought not to cripple us, but help us journey on with integrity. That is why it is the light burden.
Self-justification means proving, defending or validating yourself to others. It is what happens in the absence of self-criticism. Lacking self-awareness, many people spend much of their time trying to justify who they are and how they are before God and other people. This is an anxiety producing way of life. It is the burden of always having to ask: do I measure up? It is the burden of seeking validation every single day to every single person.
The gospel news is that God accepts and affirms us, God's love validates us. We ought to live out of an acceptance that cannot be earned. We must always question whether we are living up to that primal validation, hence the light burden of self-criticism. Instead of wondering whether you've measured up, wonder if you are living out of God's affirming love.