Sleeping with my step daughter what should I do?

Iv been with my girlfriend for almost 9 years now and she has a daughter from her previous relationship. She has just turned 18 so can legally drink and she's started going to the clubs on a Saturday with her friends. Last night she came home about 1am and she was very drunk, not incompasitated but her inhibitions were down the toilet shall we say. She was wearing a tiny little bodycon dress and I saw her hitch it up on purpose so that it was barely just covering her ass and she walked around the kitchen got up onto a stool on her knees and bent right over into the cupboard sticking her ass into the air like a cat on heat, exposing her thong which barely covered her pussy. I could tell that something was different between us and she wanted me to come onto her, it was obvious. She's an extremely sexy girl, perfect little figure and she dresses very well, but I'd never seen any kinda slutty side to her before and it really turned me on. My girlfriend had been up in bed asleep for hours and I just couldn't help myself, after brief flirting between us and some suggestive language I came up behind her while she was making her sandwich and pressed myself into her put my hands over her front and kissed her neck. She instantly turned round and kissed me and reached down to my cock and started rubbing it. I took her into the next room told her to stay quiet, I pulled her dress right up and her thong down around her ankles and pushed her onto the chair and fucked her bareback. It was extremely passionate and satisfying but now I'm feeling guilty and I'm worried she's going to tell her friends and it's going to get back to her mother. Also I fucked her unprotected and I know that she came off her period last week so that's also a worry. Half of me is full of shame and regret and worry yet half of me wants to keep fucking her.
How do I get over t of this mess
Sleeping with my step daughter what should I do?
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