If you cant even bring it up or talk about your insecurities without the worry of being "dumped" then this isn't a healthy relationship. Her sexuality and this other girl has now become the elephant in the room. If she has feelings for someone else (male or female) or is tempted to act on that (cheating) then that friend has got to go in order to protect your relationship.
Set some very clear boundaries with her. It is irrelevant that this other person is a woman. It would be no different if she were a man. She still poses a threat to your relationship and if your gf cannot understand that and refuses to do what it takes to give you some piece of mind and reassurance then you should end this relationship because living with trust issues is living in hell