Most Helpful Girl
Firstly... I have to say its kind of messed up that she is with your ex- best friend. She may not be the best person out there for you, but that wasn't the question.
Happiness is always the best revenge. It always eats me up to see my ex happy, it makes me wonder about everything. It makes me remember the good times, and how much fun we had together. More importantly it makes me question everything, including the break-up. Focus on your happiness, date other people, get involved with fun activities, live an interesting life... It's really attractive.
Most Helpful Guy
You shouldn't even think of trying to get her back. If she left you the first time for your best friend, what makes you think she won't do it again? She's not worth it, go out and find yourself someone who's worth it.