How to stop girlfriend from asking about my dirty past?

Me and my 2 year older cousin have done dirty things (kissed and discussed about sex a lot) when we were teens. Like 15,16 years old. I regret that now and is 100% loyal in my current relationship.

My girlfriend is my neighbour and my cousin once told this to my girlfriend. My girlfriend always bring this topic out of nowhere when we fight.

I've never admitted that I kissed her. I told i lied. Yes that was a mistake. I cannot tell her the truth. She'll leave me, that's sure.

Is there any way to stop her from asking this again?

(Telling her truth is not a solution)
How to stop girlfriend from asking about my dirty past?
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