We've all heard the expression and yes it is true, partially. It is true that we human beings cannot undo the past, but God faces no such limit.
In today's gospel we hear Peter questioned three times "do you love me?" And with each question and answer, Peter's triple-denial of Christ is unwound. And Peter is restored to the state that prepares him to assume his role as the head of the Church.
We human beings live in 3-dimensional space and we experience time one second at a time, moving forward. God on the other hand exists outside of space-time. It's what allows God to be omniscient, omnipresent, and omnipotent (all-knowing, ever-present, and all powerful). God is not only simultaneously present to every place but every time.
This may all sound like abstract theology until we look at how our past impacts our present. Many of us have experienced some event in the past which left us with a wound, and even when it is somewhat healed there remains an emotional scar that shapes the way we behave. If God, however, exists outside of time, then there is nothing to prevent God from reaching back and healing the event all the way down to its very source. In canon law we have a term, sanatio in radice, healing at the root.
The only impediment is us. Sometimes we consciously or subconsciously choose to hold on to our injuries. As we read the story of the healing of Peter's great sin and shame, we are once more reminded that there is nothing in us that cannot be healed if we will allow it.