Why would a girl choose to stay in a relationship if she’s unhappy?

Myself and my girlfriend have talked and we don’t seem to click well anymore and I’m ok with us going separate ways because I’m just tired of her always saying she’s unhappy in the relationship and I think that maybe she should find someone better who meets her criteria. I speak about being back on my own and she just constantly says oh well we are going through stress that’s triggering this and we can work through this. I’m ready to let go and close this chapter why isn’t she why is she so desperately clinging to a dying relationship.
Why would a girl choose to stay in a relationship if she’s unhappy?
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