Am I a horrible person, what should I do?

I've lived with my girl friend for a few years now. We have been talking about marriage. I love her but I messed up. A girl I recently met was all over me and giving me this feeling I hadnt felt since I initially started dating my girl friend. Like discovery, mystery, physical attraction. Anyways i slept with this new girl about seven times but then the lust flame burnt out and all I'm filled with is regret. I feel I should tell my girl friend that I did it but I'm scared to lose her. I have so much invested in her and I do love her I just fell into temptation and slept around. I am very attracted to my girl friend and I love her and she is who I want. The other girl is nothing compared. What do I do? I know I'm a bad guy but I need advice.