Dont be naive.
Us girls have had many instances in our teenage years in which we were less than proud of ourselves too.
What you should do is this: BE HONEST. Did you do anything that if you had heard this about her, you would break it off? Then tell her! She has every right to know!
We all went through that phase and stage in which we have made undesirable choices, but get over it! You have matured, and hopefully she will see that in you and get past it too.