Everything with his ex irritates me!

So my fiancé proposed to me about two weeks ago. But we dated after him and his ex had a battle because he was talking to me a lot. They broke up two years ago but I STILL check her instagram. She’s really disgusting looking and he tells me stories where she didn’t clean herself and got a lot of infections which I heard from many people. She’s super ugly and disgusting looking too to the point guys don’t talk to her and she’s desperate. Everything they did together IRRITATES ME. I had an ex before him and i know what i’m doing is wrong because it causes him to get really upset and have a bad day at work. But we fight about his ex and she hasn’t even spoke to him. I literally hate her and want to make her life hell. She didn’t deserve him and she is nasty 🤢. What do I do to get past this? Almost together for three years now and engaged. I constantly question him about stuff they did and accuse him of lying about it.