Me and My best-friend really likes each other like a lot but she has a boy-friend so its really sad cause I love her alot

I can relate to this very much. But I have social anxiety so my advice to you from me would be to not squander it. If she isn't happy in her current relationship, persuade her to get with you. I missed out on getting back with a girl that I really loved because I was only afraid she'd say no. Just do what you think is best, because life is too short to not at least try.

Hope I helped you! Never give up the chances you've been given. Have a nice day!