This is one of those "something is definitely up but if you ask whats up thatll just agitate them and they wont tell you whats up of their own volition" moments thats hard to handle. I think asking kohagara is a good first step but you probably need to bite the bullet and ask lemon herself, since its clearly gnawing at her.
Ideally, lemon should be able to fix this problem herself if given time, but she has a historically tereible record with fixing her own problems so maybe its best if someone helps her.
the thing that bothers me about Lemon, is that while it's all well and good to be able to rely on others, I'm not convinced that's as center a "core character issue" as her incessant need to put herself last and clam up about what's actually bothering her.
She
did have no one in middle school to reach out to, because she refused to actually talk to Sunao which lead to their breakup and complete ceasing of contact until college. But she
did find
Kogahara, who reached out to her, and then (unless I really misremember the flashback) effectively held Lemon's hand all the way through high school and got Lemon back on her feet.
Now, though, Lemon's had a huge bit of character growth, resolved that initial miscommunication with Sunao, and....she's back to square one with clamming up about her feelings of unrequited love for Sunao and her desire to not hurt Kogahara's chances with him.
To me, that's a regression of her character growth of confidence and communication, and once again she's now falling back on "I don't know how to solve this so someone please bail me out".
That's effectively a repetition of history and a nullification of ~70 chapters of character growth.
So I don't know what the author wants to do, here. Because Lemon's just going in circles at this point, in the narrative sense, only this time she risks dragging multiple important people down with her, instead of "only" her childhood best friend/boyfriend.
And Sunao's also far from faultless in this, but that's a different topic that strays from the point you'd initially made.