I think the main character is simply not nice, at all. I have way more sympathy for the girl in the first chapter, now. Even moreso if that girl is "the one" being mentioned, since the selfish main character specifically admitted she was in love with her, because she obviously was literally not kind to even that girl at all. I think new girl needs to go into it with her eyes open. sometimes a tough, aloof, jaded woman may actually be a tough misanthropic bitch, you know. they're not all diamonds in the rough.
I think the MC is just really blunt and doesn't care. The MC isn't so much a bitch as she doesn't give a fuck about meaningless relationships if she isn't invested in them. What's the point in being "nice" to someone who is only with you in a purely physical relationship.
The real problem is whether or not she lets them know beforehand that she is in it for the sex. Since she doesn't, she well... She kind of leads them on. That is the only part where I think the MC needs to change how she handles things, but she did, didn't she? She decided not to put up with the useless crap and stopped dating altogether.
And how is she supposed to be kind forever when the girl she liked was fucking any thing with a dick. Sorry but, I think the girl in the first chapter kind of had that coming.
Maybe the MC could handle her relationships better, but isn't that the whole point of a story, character growth and development?
I agree that the new girl does need to go in with her eyes open and decide from there whether to continue pursuing the MC though. That relationship will be tough.