All of this makes her honestly look super heartless and selfish
1. Realistically, it's a super awkward situation in general,
2. we STILL don't have Lemon's perspective or reasoning,
3. most people never talk to their exes again if they can avoid it,
4. Sunao never really suffered or was heartbroken until he convinced himself to be (he was sad and upset, sure; that's just part of the package of being broken up with and would happen to most people).
What did Lemon ACTUALLY do that was super heartless and selfish? All she did was break up with a boy. Are all girls who break up with boys heartless and selfish?
For that matter, no one, neither male nor female, needs a reason to break up with someone if they don't want to be in the relationship. Anyone can just leave and should if they want to.
Sure, she gave him a weird explanation and lied about why she was doing it. Does it REALLY matter THAT much? Even if it did (which it really doesn't), she was a child who didn't know how to end a relationship that she very much seemed to want to still be in, but felt she had to end due to external issues. From all the context we have available to us (which is still not much, but everything is running contrary to your opinion), it looks like Lemon felt trapped and was essentially forced to bail for some thus far unexplained reason.