Ok.
Katherine's friends say that you should go out with her. Chances are, if they are her true friends, that they are reflecting her thoughts.
Kelsey is your friend & she has been flirting with you.
What should Katherine do?
After all, you say that she is your g/f, but you are not going out with her ~ that makes you 'a free man'.
Kelsey thinks, therefore, that she has a chance with you ~ and Katherine probably thinks that Kelsey does, too.
You shouldn't be gossiping about your girlfriend to anyone ~ and gossiping is not nice anyway.
Kelsey thinks that you can't be that keen on Katherine, because you don't want to go out with Katherine, and you are friends with her, and you gossip about Katherine with her.
Katherine must think that you can't be that keen on Katherine, because you don't want to go out with Katherine, and you are friends with Kelsey, who flirts with you in front of Katherine, and you even gossip about Katherine with her.
Maybe you are too young to be 'going out' anyway, but, the way I see it, your behaviour may be making poor Katherine look a bit of a fool. Is that what you want? No wonder she's upset.
It may be possible to be friends with both girls but, if Katherine is your girlfriend, you must not respond to flirting from someone else. And you must not gossip about her with that someone else. It's rude and unfair.
