IMO - I don't think you have a choice friend. If you took her back, you will spend the rest of your time with her worried, wondering, obsessed: Was it only that one time? Where is she, who is she with, what are they doing? Why is it taking her so long at the store? Will I ever trust her again? etc. Remember, you have a life to live too. So saying, she may be a nice girl, you love her, etc. but the fact is she shot off that gun and the bullet will be forever lodged in your soul. You will never trust her again. Sure, you and she could get back together and seemingly be "happy". But are you? Or are you constantly wondering? Is this life of constantly looking over your shoulder the way you want to live?
Life is short. You should be happy. You have to ask yourself if you can let this go and cross your fingers it won't happen again, and hope that this happy will do. I can't tell you what to do, all I can tell you is that you deserve the best and happiest life possible. There is a girl out there who is waiting for a guy like you who will do you right from the start. I would think releasing myself from the drama your girlfriend brought on from her cheating would be a positive step forward to this happy life. Your decision, good luck to you. :)