You must tell her that you're not the type of guy that can be only with one girl sexually, you're just not. If she pesters why, just tell her that's the way that you are - but also mention that you really like her, care for her, and enjoy being with her. Tell her that you understand it might bother her, and she's free to leave, but you hope that she doesn't.
The fact that you admitted it here means you lost your frame. Then once she got confirmation, she ended it promptly after.
The whole point of the frame is you NOT admitting to sleeping with other girls. That is, she can and should suspect that you're sleeping with other girls. That means she is attracted to you - if she's doing this, that's good. However, she must never have proof of other women - which means if you admitted it, she got proof, and then dumped you.
You have to become a master of frame control here - deflect, or laugh, or change the subject, or tease her - but never admit.
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