Isaiah is talking to a people who know dire.
He retells a story that they know well, reminds them of what the Lord has already done for them.
For assurances of the future, we remember the past. Yet, right after the reminder of how the Lord has saved them before, Isaiah then says, "Don't remember the old stuff."
I'm reading this directive to mean that we're not supposed to dwell on our own failings.
In Lent, we confess and repent. Repent. Change the way we live, how we act, and what we say. Anguishing over what we have lost can obscure any happiness over what we have been given. Obsessing over our past failures shouldn't take the place of rectifying them.
Our new lives are possible just as new lives were possible for those ancient Israelites crossing a river while being chased by an army. I'm using verse 16 as a metaphor now. We need to get across this river to live over there--over there where our lives are not controlled by Pharaoh but rather are lived in accordance with the intentions of the Lord. It's hard and even kind of scary to change from the habits of the life we have been accustomed to--even if we don't approve of that life.
Still with the metaphor: God has the power to strike down our bad habits, addictions and obsessions.
And looking toward Eastertide, when we get across that river and are standing on dry land, we need to keep moving toward the Promised Land not go back for one more swim