Ok here's my 2 cents.
When I was 19 I had been with my then bf for nearly 3 years when I fell madly in lust for a girl I went to school with. Her and I started seeing each other even though we both had boyfriends. I started wanting to be with her more than I wanted to be with my bf at the time, she didn't want to leave her bf. I ended up rather hurt and so did my bf at the time. He wasn't able to trust me after that.
Since then I haven't been with another woman. I have been interested at times but I would never try to have my cake and eat it too again, and being happily married I basically consider myself hetero at the moment because that's what I am practising.
I think it sounds like a phase and that she does need the room to experiment. If you stop her from experimenting she'll just want to do it more. It sounds like she should let herself be single though rather than leading on these poor girls and leaving you so confused. She also should be looking for someone else who is bi, because a lesbian is going to expect her to act like a lesbian.
'People are never perfect but love can be. People waste time looking for the perfect lover rather than creating the perfect love' - Princess Leigh-Cheri from Still Life With Woodpecker.