Tell her that you want to be friends and get out of the relationship! You owe it to her to let her know it is over and not to be out fooling around, even kissing shows potential fooling around whick would likely lead to more.
If you respect her, you will end it! Let her leave the relationship with her dignity, before she finds out you were kissing it up with someone else.
PS-I am proud that you do care for her feelings (even though u screwed up a little) and you know what is the right thing to do!
Take care! :-)
Never be afraid to try something new.
Remember that a lone amateur built the Ark.
A large group of professionals built the Titanic.