Luigi didn't really like to dwell in the past at all, and if you asked him, he'd probably say it was true. But what happened in Dimension D...has never left him alone. Ever. Not even when he was asleep.
In fact, it was worse when he was asleep.
Mario had never found out, to Luigi's surprise, when Mario went into his dreams. Even Dream's Deep didn't show Mario everything going on in his mind. Maybe it was because he was there, or maybe Luigi had blocked it out of his mind when he came to Pi'illo Island for vacation. It was a time to relax, after all. (Even though that didn't really happen, thanks to Bowser and Antasma.)
Luigi never told Mario, but he actually attempted to. Quite a lot, actually. But when he tried to, he decided to give up on it, because he wouldn't want his brother to worry. But a voice in his mind contradicted what Luigi thought about his brother's feelings to him. It happened every day, and Luigi was brought down each time little by little.
He didn't know when he would lose it, but Luigi thought it would be a long time.
Wrong he was. That moment in his life knows his weaknesses, and won't hesitate to use it any longer.
Dimentio and Mr. L are fighting to return, and this time, they aren't holding anything back.