Hey! sorry to hear about your sitution, it doesn't sound fun at all.
Just to let you know, I wouldn't ever do that to a guy. I wouldn't want anyone to play those games with me, so I wouldn't with anyone else. And anyone who would, isn't worth your time...no matter how much you like her now.
There are girls out there who are relationship/committment minded...one guy at a time. If I met someone I really liked, and was dating someone else, I would have enough respect for BOTH men to break up with my boyfriend first before dating a new man. Anyone who doesn't have at least that much respect for her boyfriend, won't respect you either.
You can be nice and just tell her that you want a 100% relationship, and though you like her, she just isn't "the one". No harm in that...as a matter of fact, if it is true, then you are a nicer guy for telling her...you could eventually hurt her feelings if you don't. Who knows, she might actually break up with the man, and by then you'd be over it, and she'd be pretty upset, I imagine.
Best to just move on. Find someone with the same mindset as yourself.