There's a lot to unpack in your question and lack of pictures make it hard to answer.

sometimes I see blazers fit so good on some people, like it almost melts in with the person which allows them to wear it casually without it looking overdressed.

This depends on the person, the blazer, and context.

when I wear a blazer I dont get the feeling that I wear it casual, it feels overdressed and I dont know why?

Again depends on the blazers in question and appropriate context of the place you're wearing it. It could also be an internal mental hurdle.

Maybe have a quick look at this post on 'casual blazers' for some of the most casual pairings.