I am torn about this and I need help... :/

Well, I have a girlfriend and I have been with her for a month and a week. But the thing is, her best friend is all I can think about. I mean I really like my girlfriend, yes, but I am crazy about her best friend. My girlfriend has told me that she loves me and that she wants to be with me forever. I felt that way too, until her best friend became single recently. I feel like I am in love with her best friend. She is constantly on my mind and I just cant escape it. I liked her friend before but she had a boyfriend so I didnt so much about it. But recently, she found out that her boyfriend cheated on her and now I want to be with her. I am torn because I really like her but I dont want to break my girlfriends heart. And since my girlfriend sorta lives at my house, its really difficult. So what do you think I should do?

I would sit wait n watch,dnt make any sudden moves.Its 2 early,n sometimesu just gotta do the right thing n miss out.