How should I talk to this person?

It's ok for boys and girls to just be friends but I think you feel something more for her and I think, if you were mates, you'd always be hoping for something more. I know it's hard but I would just leave her because you really don't want to disrupt the relationship she has with her boyfriend, who I'm sure she loves. I sometimes think how I'd feel if I was dating someone then all of a sudden someone else came along trying to impress them. I don't think I'd feel too happy. Like I said, it's ok to have male and female friends but, from what you've written, I think you really like her. I know this is probably not what you want to hear but I hope this helps.