She's been working through the heartbreak little by little.
Every day that's Marco's been back and has only gone on and on about Mewni (pronounced MYUW-ni) wearing the cape, without asking about her or how she's been, her heart has broken a tiny bit more, as she's grown closer to accepting that maybe, maybe, she's not enough for him.
She kept on hoping, of course, and probably felt foolish for it. But she kept trying, kept thinking that maybe, with time, he'll come around, maybe if they have an awesome time together, maybe if she's super nice, he'll remember, and he'll come back.
She's probably been mad, in short bursts, to her diary, or cried into her pillow - but she'd already processed it so far there was nothing left but heartbroken, defeated acceptance. You saw how it was, girl, you just didn't want to believe, and now it's time to face it.