The fact you kissed someone else years ago and your girlfriend is still with you, I think you can let that one go now.

The thing is you are aware of the lying and you know the reasons why, have you shared this with your girlfriend? Does she understand the reasons? If she accepts that and she can also call you out on it, you can work together on this one. If your relationship appears to be going south, go back to basics, dating and dining. I think you can get through this if you are honest with the one you love. Go to couples counseling if necessary, if you both want to work on this.