The first thing that comes to mind is this. At any point during the dating process did anyone, meaning your Kallah or her family, make you aware of her emotional (mental) issues? In other words, were things kept from you that should have been revealed to you?
That is an important question. If the kallah had these issues before she was presented to you, and you were NOT told about it, there is no reason what-so-ever that you should have any guilt pursuing any of her friends if one of them is more suited to you. And I don’t think that her friend would have a problem responding in the affirmative either in such a situation.
On the other hand, if the Kallah developed issues during the engagement that she did not display earlier, or you knew about he issues but thought that you could deal with them, then you should be more careful how you go about it. Maybe you should go back to the original shadchan and have her make the call. Or go to your Rosh Yeshiva and have him call the girl’s parents. He can explain the situation better and he can explain that there is nothing wrong with the two of you dating.