Harry wasn't even bothered when asked.
Whereas usually, if someone else brought up his problem with Aretta, Harry would immediately get angry, annoyed, and didn't want to answer. But this time, somehow Harry could act so calmly in response to Cleo's question, which he knew was just small talk. And was not angry at all.
Instead, Harry felt that his previous thoughts were too ridiculous. Felt offended and hated when someone talked about his ex. Harry felt like he had been so childish in the past.
But he couldn't deny it. He still felt sad, hurt, and disappointed until this moment. Had not yet been able to get over her figure in his mind and memories.
Because it felt as if those times were still warm and had not happened too long ago. Even though Harry wanted to be able to forget all those bad memories so easily and quickly.