Have you read Opening Up?
Whether your girlfriend wants to get back together with you or not, it's a very important read for anybody new to poly (in my opinion).
I don't know why you lied, have you figured all of that out for yourself? Maybe you weren't clear on the rules, reading that book will help you figure out HOW people are supposed to figure out the rules for their own relationships. If truly she gave you no suggestion that honesty was vital to your relationship continuing (or never really got around to giving you clear guidelines for what poly means to her and what she expected out of your relationship), at least reading this book gives you a segue to open up dialogue with her again - you did not know just what the expectations were, after you read more you realized just how and why what you did was so crucial to your relationship not being able to continue, and you understand better and hope she will be willing to talk to you about it. (And no, if you think you were right at all to keep things hidden, just bow out gracefully would you?)