If you are "split up" or you "break up", you are still seeing other people. If you feel guilty, tell her you want to see other people, and leave it at that. She might never want to see you again, but at least you were honest. Don't have a "secret fling", just be honest about it.
You should be really careful and really think about seeing other people. Are you sure you're ready to settle down with just one person? Does she really have the potential to explore new things with you? Is she an explorer? Does she like adventure in the bedroom? Because it sounds like you are just beginning to explore new part of your personality, and sexuality. What I'm getting at is, I think you might need a girl who is ok with an open relationship and you need to figure this out before you get married.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
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