Jealous of his ex

So this is going to sound so pathetic and it really is and im aware of how toxic it is.... but i am jealous of my boyfriends ex and tbh i dont know why sometimes. I’ll give you the back story.......

He met a girl online and he was with her for a month or two (they never met up as she was from another country) and when they broke up he was in pain for months until he met me. We met in june and fell in love and before we got together, he used to bring up stuff about her. Talk about all the stuff they did together, some tmi stuff I didn’t need to know and told me that he had never felt love like it with her. We have been together 6 months and he always tells me that our love is magic, nothing else will compare. But after him bringing her up alot i always used to compare myself, I didn’t feel good enough because I wasn’t her. And i felt like i was copying her and their relationship because we do all the same things they used too. Im fat and ugly and she is slim and i wish i was slim.

This all sounds ridiculous. I know he is with me for a reason and he hates her and has not spoken to her in almost a year and wants nothing to do with her. We have done everything they never did, hold hands, cuddle, kiss and he lost his virginity to me. He wants to marry me within the next year and a half and we are moving in together in may.

How do i shake these ridiculous and pathetic feelings of jealousy and insecurities? How do i stop being jealous of a relationship that is in the past? How do i stop being jealous of the fact he was in love before me?

I ask myself these questions all the time and i dont know what to do. I feel like a horrible person and girlfriend, i feel so crazy :(