“It is what it is” means what happened in the past is real and cannot be changed no matter how hard you wish it could be.
Remember, even if what happened wasn’t your fault, whining, crying, and complaining serve no purpose except to frustrate you and waste time that should have been spent trying to figure out how to prevent the same thing from happening again in the future.
But remember what Rafiki tells Simba in The Lion King, “Ah yes, the past can hurt. But the way I see it you can either run from it or learn from it.”
Nothing lasts forever and time heals all wounds if you choose it to.