No matter how hard Mutsumi thought Taiyo was pushing himself...he was actually pushing himself way harder
. I get it though. He doesn't want to wait the amount of years it would take to slowly move along. Now that he's decided to be in...he's all in! His own sense of trauma is probably fueling his panic and desperation. He's already lost it all once. Losing Mutsumi would utterly destroy him so he's got to do whatever he can. Or at least he feels that way. But pushing himself until he's broken...will simply break him.
I do like seeing that flashback. They weren't all that close, just neighbors. Although must have been a wild thing to live anywhere near that family! Sounds good to me that Mutsumi just ended up falling for him when he stepped in to help her out. Someone outside of her family stepping in for her meant a lot
. And all the more so when she really must have had mixed feelings over that streak of her hair.
Thus she was there for him when things went unbelievably wrong
.