HELP

Okay so I am currently 16, going into my junior year. This past year I dated a freshman ( very clingy). Well about 3 months ago, I was home alone my mom went to a different state to visit a friend. That night I called this girl, a senior, and asked if she wanted to come over, she said yes. At this point you're probably like wow you were, well cheated on your girlfriend. The senior came over after work at about 10:30 pm, let's just say she didn't leave until 1:00 am. Anyways out of guilt I broke up with my girlfriend, and she did not take it easy at all. Three months later she is still calling me and texting me non stop, I was wondering if there is anyway at all that i could show, or tell that her and I are over and it is for the best?

Misc.
-I know cheating is wrong.
-Any advice will help!