Love a bisexual girl

Hi all, new to this forum and would like to get something of my chest and hopefully get opinions on my next steps with you all. Before we start I'll make it clear that I'm male just in case the story confuses anyone. For at least a year I've had very strong feelings towards this girl to the point I'm fairly certain I'm in love with her. We're close maybe even best friends close and I enjoy the time I spend with her. I want nothing more than to tell her how I feel and for her to feel the same. I know it sounds simple enough but unfortunately she is currently in a relationship with another girl who she'd been best friends with for a long time before eventually getting together. The girl she's seeing has cheated on her more than once with men but the girl who I "love" has said won't leave her even if she carries on cheating. This really hurts me, every time I see them together or even think about it I can't help but think that they shouldn't be together and she'd be happier with me. I'm not the kind of guy to try and ruin someone else's relationship to get my way so I'm letting it run its course, plus me trying to convince her to leave the relationship might cause some arguments. I'm hoping that one day she sees the other girl for what she truly is. I can't stop thinking about her and it has got to the point where I'm frequently unhappy, in the back of my mind I want to forget about it and try and find someone else because I'm sick of chasing a girl who I may never actually have a chance with but these aren't just feelings you can shrug off. How can I get past this, what can I do? Any help would be greatly appreciated, thanks for taking the time to read my problem.