Simon isn't surprised that Blue figured out his identity, but he feels horrible for thinking that Cal was Blue. All through the morning, Simon moves like he's in a daze. He keeps thinking of Blue and by the end of the day, when no classmate professing to be Blue has come forward, Simon feels heartbroken.
When Simon feels horrible about his assumptions, he gets to experience a small taste of the guilt that Martin is experiencing in relation to his own assumptions.
On Thursday, Cal casually mentions he's bisexual and suggests that he and Simon hang out. Simon can barely respond. He forgets all about it until the next day in English, when he walks into the classroom to find Nick on Abby's lap. Nick wants to know all about Cal, and Simon doesn't know how to explain to them that he's kind of already taken. Blue, however, hasn't been returning emails, and Simon thinks Blue seemed to like him better before he discovered who Simon is.
Here, Simon's unwillingness to let any of his friends in on his relationship with Blue means that Simon feels even more alone than ever. However, it's also important to notice that Simon believes Blue isn't returning emails because he now knows who Simon is. This is a pretty self-centered belief, as it ignores that Blue may just be upset that Simon didn't figure out who he was.