'Can it be possible that my mother was actually a superhuman?'
According to Jack's understanding, he knew that if a person really wanted to isolate a certain type of memory, then that person had to actually volunteer to do something that was going to enable them to be capable of doing it.
That person would have to do something like perhaps avoiding something that will remind them of that event, try thinking of things that were completely opposite of what they were trying to forget and so on.
But no matter how many times or how hard he tried remembering, there was not even a single day that he had actually been made to try to forget about Celine.
And even though he had already recovered his memories about Celine, he didn't even remember how he had lost the memories. To him, he could remember the day that the two of them were parting ways, and ever since then, they had never come into contact with each other until they had grown up.