Mamimi's "So long Naota" cemented the shock that it was over and I had to lie down on the couch until I saw Haruko's guitar give one last twang before the mood whiplash (yet oddly fitting for this moment) of the credit song Ride on a Shooting Star.
The manga is one hell of a tearjerker. Haruko leaves Earth, having left behind her Vespa scooter for Naota to follow her in the future. However, the modified scooter is far too difficult to handle for him, and drives so fast that his fingernails start to rip off. The final pages show him trying again and again, crying his eyes out all alone whilst cats watch him.
The ending itself makes you want to cry when you see him grow up.
From episode 6, after Haruko's return, in Naota's room:
Naota (to Haruko, crying into her chest):"Where did you go!? You left without saying anything!"
Not at all helped by the ambiguous expression on Haruko's face at the time. It's impossible to tell if she actually cares about Naota or if she's just using him.
Let's go back to the first episode when Mamimi finds out Tasuku has an American Girlfriend. Break the Cutie indeed.
Also, this troper almost teared up in "Brittle Bullet" when Mamimi rejects Naota's advances and kiss.