The end is so sad, she's alive just because she love and not for herself, that's just a toxic relationship...
Those seem like an unreasonably pessimistic takes.
It can be very easy to develop an extremely negative self image with certain neurological and psychological disorders, both because of the direct effect they have on your life and because of the way society treats them. This is especially true when you are otherwise competent or even very intelligent. Having someone who can help compensate for some of your issues and remind you that your not trash can be incredibly helpful.
My read was that the MC was very socially isolated growing up and her girlfriend was basically the whole of her support network. That's not particularly healthy, but the issue isn't a toxic relationship. It's also not so much a matter of living because she's in love, rather than for herself, as her lover giving her the tools she needs to live.
Okaaay... so the moral would be something like this:
If you are a lesbian surrounded by het girls and are desperate for female companionship, go after the one girl in school who is completely alienated, depressed, has zero self-esteem and may have mental issues. Even if she's straight, she will be so happy she'll agree to anything you want ─ and will even move in with you after you both graduate, as by that time you'll have become her hero, her goddess, and the sun of her life.
Eh, there's worse. I'm not complaining.
I'm not even sure where this one came from. There's no indication that the Senpai chose her out of desperation or that they do not genuinely care about each other. Quite the contrary, the Senpai seems to view her quite highly, despite her issues.