When should I tell a guy about my past?

I used to be a lot more promiscuous and I used to date a guy who convinced me to do things that I'm not proud of. He liked to see me have with other guys and do gangbangs and stuff like that. I left him and I don't do that anymore. But I feel I need to tell guys I date about my past, but I'm not sure how. I've had 7 bf's this year, and each time I tell them they either dump me or back away emotionally and only still want . I know I was slutty before, but that doesn't mean I am that now right? I want to be honest and it's something anyone I'm with should know, but how and when do I tell them?