“Ought implies can” in that linked article is about the present and future, not the past. There is nothing in that principle to disallow having a preference that the past had not been as it was, and to have regret for former actions. The past cannot be changed, but one can learn from one’s past errors, and strive to become someone who would not have made that error, and so will not in the future.
You’ll find it helpful to ignore that aspect for now.
“Ought implies can” in that linked article is about the present and future, not the past. There is nothing in that principle to disallow having a preference that the past had not been as it was, and to have regret for former actions. The past cannot be changed, but one can learn from one’s past errors, and strive to become someone who would not have made that error, and so will not in the future.
It only implies that you can have no moral imperative to change the past. It has no consequences whatsoever for morally evaluating the past.