If you are both in a place where you feel like a stable relationship is possible again, then it is best to wipe the slate clean and forgive each other for your past actions. It is up to you whether you feel like you need to confess to your girlfriend about the one night stands. On one hand, if you were in a pattern of break-up/make-up, then one could be confused as to when you were together and when you were not. Being in a relationship where your significant other is consistently unsure how they feel about you and if they want to be with you is extremely disheartening, and you cannot blame yourself for doing things which exhibit your confused behavior. On the other hand, in order to start fresh, you both should talk about your feelings and how the rough times affected you. If you are feeling extremely guilt ridden then it is probably best that you confess, but explain how you felt at the time and how things are different now, and reassure her that it will never happen again. From what I gather, she has seen other people as well, so as long as you are both exclusive now, that should be all that matters.