Until the author properly explains why Lemon did what she did, everything she says will always fall flat. We know they are going to end up together, it's just how these things work, it's just a matter of how long the author can squeeze juice out of this particular apple, but all these "I loved him, and I do, but then I did something ... (incredibly super cruel that made him believe that I not only didn't love him but cheated on him like it was nothing)", just do. not. work.
You don't start your manga by making the heroine out to be a complete bitch and then expect sympathy.
First of all, we are human and flawed. Adolescence - the time when the events of the manga probably occur - we will always be "knocking on the door" for nonsense.
In fact, until now there is nothing clearer about Lemon from the past (except her love for Sunao). There's something a little clearer about Sunao: he's another one of those who hasn't yet learned how to interpret feelings and "fell" for an exaggerated lie from a girl who probably just wanted to "take a break" from the relationship and didn't know how to talk like that. If she was influenced by a friend or a gossip magazine, she will know (until the time the author talks about it). It was Lemon's mistake and she still doesn't know how to correct it - or explain herself.
In addition, the confusion events occur during the exchange of bodies. And the funny thing - it's a romcom - is that they don't talk about their feelings with each other. If they talked, the story was over and done with.
But well, romcom is that thing: we talk about people who don't know how to deal with their feelings and get into a spiral of confusion. Sunao should have learned more during the exchange (and Lemon too), but he didn't. Let's see how far it goes, and I hope the story is good until then.