Sorry to say that but when you come out, your friendship will most likely be over as well. You can't just admit that you don't know whether you're even attracted to women after a relationship with her and expect there not to be emotional fallout.

I don't think you can get her to be super supportive of you either, even if she is pro LGBTQ (although I doubt that if she thinks it's a sin - you can't be pro queer people and think that we go to hell, just saying). Her own hurt over the end of your relationship will understandably play a bigger role for her than your feelings.

As a fellow queer person, I don't think you're to blame for wanting to break up though, especially at 17. Sexuality is a weird thing and it takes a while to figure everything out.

But I still think the best course of action is to give her a clean break. Tell her you're unsure about your sexuality and that you're breaking up with her to find out what you want. Afterwards, don't contact her unless she explicitly wishes to stay in touch.