This is a rough one. I can sympathize with both characters here, honestly. If you are the type of person who needs a relationship to be at their happiest, while also being a lesbian, the world of romance can be incredibly unkind, especially if you have any other qualities that makes you more unusual or less desirable in the eyes of society. You end up clinging to a relationship that mostly works, ignoring the parts that don't work, just because you want something to not go sideways for once. Just because the happiness from a relationship that almost works is much better than the way you feel when you're alone.
And then there's Mio. Somebody who values her friend highly and questions her own feelings but is willing to sacrifice her own happiness to grant happiness to someone she cares about. She thinks hurting herself this way is noble and does it up until it finally breaks her. I have been at this level of despair a lot.