@The_qwerty: On the surface, what Mone-san was saying was what you said. However, what she said in addition was clear, there was no hidden meaning involved: "Tell them your feelings when you have the courage."
Not to the ears of affectionate feelings, at least.
Indeed, Tsubaki swallowing and suppressing those feelings of hers will only lead to them rotting her from the inside out--it shouldn't be done. The way to get past those feelings is to face them, acknowledge them, and
let them go. This requires that those feelings themselves be rejected and thus denied. Now, once again, the guy she
likes is in a committed, happy relationship. It takes guts to wedge yourself in between two lovers in an attempt to steal one away from the other; indeed, if you're a man, and you are attempting to do this to another man, you can expect a violent reprisal. If a third party has the intent to inform a pair of such lovers that said third party has feelings for one of the two, it's not without a reason; no matter how the reason is dressed up, no matter how much the person in question denies it, the reason exists. That reason is, of course, that third party's desire for that one lover--the very act of such a confession is, in reality, embracing and feeding the feelings that gave rise to the confession in the first place.
Simply put, going so far as to tell
Mone-san's boyfriend that she's still got the hots for him
in spite of his relationship isn't "confronting" her feelings at all--she'd be holding onto them, because one doesn't confess to this sort of thing because one wants to end up without the person one confesses to. Furthermore, Tsubaki would effectively be attempting to wedge her little "problem" in between Mone-san and Nobuyasu for that implied purpose--hence why she said what she did about interference (she knows it's wrong). Speaking of, even if the couple is a strong and happy one, there's no telling what such an action by a third party could do, and no normal couple would appreciate a third party trying to steal one or the other away.
So Tsubaki was right: those feelings are exactly what she needs to discard. She has to realize that she did in fact lose (even before putting her bid down), and give up; to do otherwise is to bring harm to herself and/or Mone-san and Nobuyasu.