I think you should ask her if she's a bi-sexual, as it might seem she's the same as you, but she might just like you as a friend, or a best friend. You both are friend's, right? Tell her that you like her, and ask if that's alright with her. If she think's it's 'gross' and 'wrong', then really, she's not really a true friend. But she might even like you to, but want's to stay with her boyfriend, so other people won't think of her as weird, or wrong to be different.
As for the boyfriend part, don't mind that, just tell her, and you'll never know, she might even like you back! And if you told her in the past, that your bi, than she might be okay with you liking her. But if you never told her, than she might not like you the same way. But I hope this helps!