Help.

I have a hard time forgetting about the past. It’s also been making me have problems in my relationship. I’m just tired of thinking about how many girls he slept with and about his ex. I don’t know why I think like this it’s a fucking headache honestly. The reason why I think about his ex so much is because he told me that she was his first love and she was a virgin and other stuff like that.. I have insecurity issues maybe that’s why I hate her so much I don’t even know the girl. Like wtf is wrong with me??? I really don’t want too loose my man he’s a really nice guy and have supported me in every way since day one we been together almost 6 years! I don’t want to ruin my relationship. I know I love him and I want to be with him but I do t know what to do anymore. Any advice you can give me would be good.