You should tell the truth, but most likely only lies will get her back. She left you in search for something that is not "you". It doesn't matter what you did, it's what your willing to do. Are you willing to accept yourself?
It depends on the reason she left. Girls are complicated. If she left you dude she never loved you. Once a girl loves she never leave. mmmmhh, unless mmmmh Did you fuck another girl? Then I am sorry might not come back but if she does shes a keeper.
The question would be, "What did I do wrong?" You need to know exactly what you did so you can say truth about it, and then when you get her back you'll know not to do it again.
