Adachi obviously comes across as kind of a crazy ranting loon in this... confession? Rant? But I kind of like that for once it's not like a sparkly beautiful stereotypical confession scene, she's not even fully aware of what she's saying it just all comes out. Some of it's on Shimamura too, she never responds so Adachi just keeps on crying and talking.
And it is tough on Adachi... She knows she doesn't have the right to be jealous because she's just a friend, but also it kills her that she's still just a friend, and she's been kinda hoping that OK well, technically we're just friends, but really maybe we're something beyond just that, we just haven't said it openly yet. The way Shimamura always goes along with everything but also never really provides anything to hold on to really is kinda brutal - Adachi and Tarumi kinda need to form a support group together, they're both having the same struggle!
And there's also obviously other shit going on for Adachi too. The series doesn't really focus on gay angst but it's stuck around under the surface, and she's obviously freaked out Shimamura would hate her if she's too open with her crush.