I have a girlfriend who care about me a lot, but I am in love with my best friend. WHAT SHOULD I DO?

Take a break from your girlfriend AND your best friend. And decide whom do you actually want to stay with. Does your best friend has the same feelings for you?

Ah, the sounds of immaturity. You are being selfish if you don't cut the ties. Is it that you are hanging out with your girlfriend until you see what can happen with your best friend? It is called selfishness. Let your girlfriend go so she can find someone that truly cares for her. You obviously do not and staying with her would make you a user.