That in regards to the ending, the way I see it, it's not so much heartbreak, than it is disappointment. It's his first ever "relationship." He doesn't know what that feeling is. He even literally asked "is this what heartbreak feels like?" No, it is not. Heartbreak is followed by depression. As someone who was rejected and dumped a few times already, I'd know the difference. In short, he wasn't that emotionally invested in Miki. That, again, is the way I see it.
Y'know, I think we skipped a few mile-long steps, but hey! We finally got a love confession! Yay(?)!
This comment is too accurate