Am I a horrible person?

I had feelings for a guy for couple of months , but lived in different cities , i knew he had a girlfriend but my feelings for him ddint chgange. I later on moved to thge same city to start my university , me and the guy became a lot closer and he still had a girlfriend. The girlfriend was in a different city , and they started fighting a lot cause i always used to hang out with him and she knew how i felt. Eventually he told her that they were going to break up in 3 months from now , cause he couldnt handle long distance and their relationship started to change. She stopped coming over as often and started treating him badly , because of that he came to me and said we should start having sex discretly till they break up. I agreed to do so , till they broke up. I knew the girl liked him a lot , and i was part of the reasons why they foughtg. Up until now , i never felt guilty. I never liked being the side chick , i ended stuff with him occasionally because of it but still went back to him. It just dawned on me that i am a horrible person , she later on found out because i made him tell her. They were going to start a relationship again but it didn't work out. I am currently with this boy , stuff later ended for them and grew for us. Right now i can't forgive myself for what i did to that girl , i wish i never did what i did. Sometimes i get angry at him , and myself at most. I can't even look at myself cause when i do , i see a homewrecker , a girl that took someones boyfriend from her. I am 18 years old , i started sleeping with him when i was 17 back then everything felt so rosy , but when i think of it again i just feel guilt. My question now is do i leave the guy , i really love him and things are working out well for us , but as the right thing to do. Should i leave?
Am I a horrible person?
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