That...was honestly kind of a dick move on Mitsuko's part. I mean, she knew about Mako's feelings for her for HOW many years? Why didn't she just confess herself? She's clearly assertive enough! But no, she's just so hungry for a confession that she allows the girl she supposedly loves to go on thinking that her feelings are one-sided.
Also, did she actually date other people in the meantime, or was that breakup she was whining about just another way for her to try and trick a confession out of Mako? I'm honestly not sure which of the options is worse.