I think it refers to the big earthquake.
It says:
We have the ability to forget
We have the ability to remember [literally: "to be without forgetting"]
But,
There are things we are unable to forget
(Or if you want to keep the continuity of using 忘れる ("forget") three times:
We have the ability to forget
We have the ability to not forget
But,
There are things we are unable to forget )
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