I know I'm a month late to the party, but hear me out...
Nobody is perfect, neither Ito nor Sumika. All of the people here in the comments going, 'uwu how could Ito be so mean to best girl Sumika what a bitch,' clearly have no clue about the hardships either of these girls are facing. Both of them are contending with very different and distinct issues that, when clashing together, explode in the way we see play out. I'm willing to bet money that almost every person in this comment section wasn't some paragon of their own feelings at sixteen; don't forget that these two girls are teenagers. As a lesbian, I can guarantee to you that navigating the sea of your feelings, especially at this early of an age, is exceedingly difficult — especially in a society like Japan where so much pressure is put on young girls to conform, be demure, and endure their pain for the sake of others.
Sumika is lonely, yes, but more than that, she's selfish and doesn't consider anyone else's feelings but her own. Her confession in the pool was an inherently selfish act, and at no point has she ever stopped to think how Ito may feel or what she may be going through. While you can understand and sympathize with her desire to be close to the first person to show her companionship like that, at the end of the day, Sumika has done nothing but force herself — and her feelings — on Ito. That's not to say that there isn't growth from this; it's apparent that this is the point in their relationship where the two part ways for a bit to sort themselves the hell out.
Ito, on the other hand, is dealing with a lot of issues, seen and unseen. There's a very common saying passed around, 'Don't meet your idols.' More than that, Ito has done that twice — with her former senpai and Ito. Both times she's been let down. There is a particularly profound and distinct pain to know the person you've been looking up to you your entire life thinks and talks of you as nothing more than a loyal puppy following them around. To Ito's senpai, she was nothing more than an ornament, an object to be touted around and shown to her friends. And Ito found that out the hard way; that sort of knowledge is absolutely devastating, especially when you've all but dedicated your life to that person. At this point, I wouldn't be surprised if she loved her senpai, which serves as another point of contention with Sumika — who at their very first meeting, did the same thing that Ito's senpai did.
This isn't meant to absolve Ito of guilt, as she has absolutely taken her anger out on Sumika to some degree, but to explain her situation. Sumika, more so than her senpai, is Ito's idol — a perfect, unachievable being whom she can look up to. The 'Sumika' she knows isn't a real person, both literally and metaphorically. To Ito, Sumika would never confess feelings for her, because how can someone like that love someone who feels themselves to be as wretched as Ito does. In her mind, the 'Sumika' she's idolized can't be a real person, because that 'Sumika' is perfect and wouldn't fall in love with someone like her. She's not saying Sumika doesn't exist; she's saying that the idea of who 'Sumika' is in her mind doesn't exist.
And that hurts. That's hard to deal with, especially when you've idolized a person like that. Moreover, in the last chapter, when she pushed Sumika away, she was entirely justified — if overly harsh — for scolding her. Sumika used another person's heartfelt feelings as a way of reaching out to Ito — and not that person. While it may seem sweet on the surface, it's just as Ito said; she gave that person hollow words that weren't meant for them. To say that 'ew Ito is so mean she's such a bitch' and 'uwu perfect best girl Sumika' is a gross infantilization and misunderstanding of their characters.
Finally, the last thing to note is her rejection. "I don't like you in that way at all." In Ito's mind, Sumika and 'Sumika' are two different entities. Which 'you' did she mean? I'm not sure what the raws were or if this is an adequate translation, but the way I interpret this isn't as a straightforward rejection. In essence, the 'you' she's rejecting isn't Sumika herself, but the idea of who 'Sumika' is in her mind.
tl;dr, both girls are flawed; both are making mistakes; both have a truckload of issues; stop calling the characters shitty for not being 'uwu perfect gay bean waifu.' It's not the writing that's shitty; it's your expectations.