I fucked up how do I make it up to her?

I cheated. Back in May 22, I performed with my band. Before we performed for the crowd, I had a couple of drinks with a few people I know who are part of the crowd. I wasn't emotionally well and then this girl comforted me. We laughed at my problems and sorrows. One thing led to another and she kissed me intently. My girlfriend and I had a fight prior to my gig. I was mad and I was drunk. So I returned the kiss and eventually we had sex. I didn't enjoy when I performed for the crowd and after we finished, I went to her and expressed my frustrations. She said I shouldn't care because we both wanted it and that we'll never see each other again. And then I saw her again yesterday at my gig when I was still performing. She just stood and stared at me. When I was done, she approached and complimented my performance. I was a little scared because why was she there? I could smell the alcohol in her breath. She hugged me but I pushed her away. I told her that what happened between us was a fucking mistake and that I was drunk when it happened. And now she's blackmailing me that she'll tell my girlfriend. I wanted to forget everything that happened back in May 22. I didn't tell a single soul. I'm terrified. She knows my girlfriend because of my social media. She said she'll expose me. I don't want to lose her because of this fucking bitch. I know I fucked up back in May 22 but I regretted all of it. I promised I'd be better and I am better right now. She said she'll tell her if I don't treat her. She wants me to help her with her job and her schoolworks. If I don't, she'll my girlfriend. Please, I know I screwed up but I AM BETTER NOW. How can I get away with this? I don't want my girlfriend to know. I just want everything to be normal again. I want to get away from this.
I fucked up how do I make it up to her?
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