Uh, yeah, you should be honest with him. But maybe even more important, you need to be honest with yourself. What’s going on with you that you would cheat on your newly-minted fiancé, not just once but twice? Do you have a history of cheating before these instances, or did the cheating start only when you got engaged? If it’s new, what is it about being engaged that is freaking you out? Are you afraid of commitment in general, or do you have some doubts about your fiancé, specifically, and whether he’s the one you want to spend your life with? Either way, you need to address these issues you’re having, discuss them with your fiancé, and figure out together how you want to proceed. Maybe you need to put the engagement on hold. Maybe you need to actually break up. But one thing you definitely need to do is stop fondling and kissing other people when you’re committed to someone else.
P.S. There’s a huge difference between flirting and lying with someone and fondling him. Again, you need to start being honest with yourself here.
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