Well, you should go away for a while, or focus on your career or something...
One thing to never mess with is a woman's friendships, trust me, those women would usually rather be without a guy & hang with their best girlfriend for life, than Cat-fight over you. Ever heard the phrase "Chicks before Dicks?" Yeah, I'm sure.
This isn't anything that any man can change, it's just female nature, so I suggest you hang out with both of them, only when with fellow friends, Only in large groups.
I have tons of Female Friends who have boyfriends, But we Don't talk about anything inappropriate. I simply look at them as Other human beings who I cannot take interest in, Trust me, You DO NOT want to cros that line. It will make you look like a cheat & a loser.
Don't hang out with her (the girl you like) alone anymore, or else it will cause suspiciton,. NEVER be alone w/ her , especially when your girlfriend's around. Here's the thing, try to get the GF to break up with YOU, then it will be better, maybe giving her reasons, besides being a bitch about.
Furthermore, don't do something that you know is morally wrong to get her to break up with you, like disrespect her, break committments to meet, etc. It will make you look like a Shady persona, and since Females tell each other EVERYTHING, yes, everything, even your size,, the HB 10 will see you doing the same thing to her as when You make her Your Girlfriend.
Back to the girlfriend:
Although, don't do something like "I'd rather be single right now" and then hook up with her best friend, 'cause then you'll look like a hypocrite, If you'd rather be single, then why do you need a relationship right after?
it screams "REBOUND!!"
Furthermore, propose that you guys go out a lot with your friends, maybe having an open relationship instead, but DO NOT go out with her friend at this time, give it some months, & I'm sure she'll be yours
I know, it's a long lecture, but you need to watch this, it's long, But There Are Friggin' Diamonds in this advice, watch it as many times as need be as well as the accompanying videos.
Good Luck[/i]
http://youtube.com/watch?v=NdNNZ905wGA
You could always tell her you wanna explore your options.