When I left I didn't really feel the need to go through doctrinal issues and prove it was wrong. Not everyone processes everything the same. Part of loving someone I think is giving them the space to process things however they want.
I think people are more angry depending on how involved they've needed to be. As an elder's daughter, I'm sure there was enormous pressure to conform and you were constantly doing things you didn't want to. That can create alot of anger. Personally, my mom was a single mother so I think expectations were low on how much I participated. My family wasn't looked on as an example so I was able to mess up (as kids do) and it wasn't terrible. Anyhow, I wasn't super angry when I left - everyone's experience is different. Don't discount his or minimize your own. You know how sometimes siblings in the same family can have completely different perspectives on their upbringing? Even when they were in the same house with the same parents? It's kind of like that. And yet each perspective is valid.
Good luck- I'm not a person who advises getting married young- I made that mistake and divorced but it's nice to have a best friend with you as you leave. It doesn't have to be an all or nothing scenario. Just remember you guys can leave and be boyfriend and girlfriend (outside the religion) for a while and see how that is.