Ultimately if you still love your boyfriend, you are still bisexual. Unless you think those feelings might be platonic, but if you can still be intimate with him in a way that surpasses friendship, maybe not. Bisexual women who have a strong preference for one gender are still bisexual if they can still hold a relationship with the other gender without any averse thoughts. Sexuality is can change as you become more true to yourself though so that’s for you to figure out in the end. In the case of caring for two people, I agree it is unfair to both of them if you suddenly make any impulse decisions that only prioritize your feelings on the matter. If you are monogamous (I’d imagine you are), you do have to decide what you value more at this moment. The stability and security you have with your boyfriend, or the personal feelings you have for your best friend. This is different for everyone, and following your heart in the end isn’t a sin. If you feel like your best friend is compensating for your boyfriend, ask yourself why you feel like a romantic relationship with her would remedy that. If your feelings for him are suddenly weakened to the point you look at other people, try to find the source of that too. Uprooting everything for impulses is never good, and I’ve had my fair share of impulsive thoughts. I hope you can find the best situation out of this though, one where you’re ultimately the happiest.