It's not easy dealing with rejections I guess.
It is not.
Even so, she prepared herself for that for a long time. Wasn't it her who said she would rather share her feelings and be done with it than to keep holding it in? I'll be sympathetic only so far...
Sympathize however you see fit, but as anyone who’s ever gone through a “I really value you as a dear friend” situation has experienced, there’s no real “preparing” yourself for rejection—there’s only putting off the inevitable.
I went through that myself once (although not as severe), so I understand that much. But in this particular case it was sheer impossible and also way more problematic for the one who was confessed to. Uta is being incredibly selfish after all. She can grieve over it as much as she wants, I just would prefer she did not hurt Kaoru more in the process.
Imagining something isn't the same as actually going through it. It seems that Kaoru brushing off her feelings so casually hurt Uta much more than Uta expected it to. And to top things off their horrible mother shows up and starts messing with Uta's emotions even more which causes Uta to be unable to control her outbursts.
There are certainly many factors putting pressure on her at once, I admit. Everything related to her mother is fair game from what I've seen.
It would simply not do to make Kaoru suffer for Uta's selfish love though. She needs Uta more than she lets on - but as a sister. If Uta really loves her, she should understand this eventually.