Your letter to me starts by saying that you ended a relationship and then you go on to say that she left you for your best friend. It sounds to me like you were dropped and I hear that you are sad, which is a normal reaction; but what I don't hear is how mad you are to have been violated in this way.
The first step to getting over her is getting mad. But, as you will see, there is more that will need to be done so that you are really free of her and immune from falling for another girl like her. In addition to asking how you can let her go, notice that you also asked me how to get her back. Clearly you are conflicted. Part of you wants her back, and part wants to move on. Before you can move on, you must understand what part of you wants to hold on to someone who mistreats you so badly.
There is only one reason that I can find for your yearning. I know you think that it's love, but love isn't what makes you want this woman to come back to you. It's unfinished childhood business. Stay with me now. It is human nature to seek pleasure and avoid pain. Notice that you are doing the opposite; in wanting to get back with her, you are looking for bloody nose. The question is why? The answer is actually simple. When a person is abused in childhood, his/her mental wiring gets all screwed up.
The abuse programs us to think that mistreatment is normal, the way life is supposed to be, and in no time we are seeking partners who give us pain instead of pleasure. People who were abused as kids find lovers and spouses to abuse them partly because they think that this is the way life is supposed to be and partly because they want to relive their childhood and achieve a happy ending this time around. To be sure to have an accurate recreation of the original wound, we must choose lovers or spouses who are damaged and limited the way our parents were. Soon we are right at home, feeling mistreated in the exact same way we were mistreated as kids.
And, here's the kicker, instead of running for the hills, we stay, can't let go, want the monsters back, not because we love them but because we are hooked on the hope of healing the original wound, hooked on the hope that this time mommy or daddy will love us, respect us, admire us, not leave us or whatever else we needed as a kid and didn't get.
So as you can see, you are hooked on a recreation of your childhood. This girl has abandoned you and you want her back because you hope that this time around you will be so good and so loving that you will win her love; and when you do, you will feel like you succeeded in making your mom or dad love you. You need to see that because this girl is damaged goods, she isn't able to give you any more or any better than your parents did. It's normal to keep hoping to fill the void inside yourself, but you need to tell yourself that it isn't going to happen by having her back in your life.
So, to able to let go of her, you are going to need to heal the wound inside yourself. When you are healed, you won't want anything to do with her or anyone else like her. To heal the wound you are going to need to reparent yourself. Buy some books on inner child healing. John Bradshaw's books are a good start and will provide you with exercises for giving to yourself the kind of parenting and love that you needed and didn't get.
Also join a therapy group, which I always call your 'second chance' family. There you will make connections with people who give you the love you have been craving since you were a kid.