Hopefully she won't leave you, but I can tell you with absolute certainty that she will if you break her trust again. I wish you luck my friend.

You can't just tell her that you won't do it again, you have to be able to prove it over the long haul. She may or may not be able to forgive you and move on, but it's not going to happen overnight.

You hid a relationship with another woman from your girlfriend. This is a betrayal of trust. Unless you realize that you were in the wrong to have done this, and are truly remorseful she will probably never be able to trust you again. If this really is the love of your life it's time to man up and admit that what you did was wrong, then all you can do is hope that she'll forgive you.