Saki's self sabotage is a bit extreme.
the last two chapters have generally felt a bit extreme to me, or is it just me who thinks that Saki is completely exaggerating way too much?
Saki is still a child in school, and a girl who's a lesbian on top of that, struggling with very intense internalized homophobia.
She perceives everyone as out to get her because of this, especially in the case of Mahiro, but her perception was challenged when she actually met her again.
I don't this it's at all "too exaggerated" for someone like Saki to feel like she's truly feel as conflicted as she does, because her worldview has almost been completely shattered.
You can obviously see the progress being made here: she realized her Mom wasn't actually being dishonest or hiding things from her, then that Mahiro just wanted to be closer with Saki, and finally that Kanon also loves her. Saki never imagined any of these would contradict what she's always believed, so this chapter is the culmination of that, where she is coming to terms with the fact that she is the only one who's been holding herself back the whole time.
So, yes, Saki has been exaggerating a lot, but it's important because it's the climax of her self-actualization not just about her feelings for Kanon but also everyone else around her that she projected onto. This has been built up for a while during this arc so I don't think it's really out of nowhere, either.