Ok, I found the
ser vs estar thread I was looking for. I have also now stuck it to the top of the Grammar forum so it's easier to find.
That thread should help you to understand, but I'll also explain it some more here. Ser is used for definitive characteristics even though they are not necessarily permanent:
Soy nuevo aquí - I'm new here
Soy fumador - I'm a smoker
Soy alchólico - I'm an alcoholic
Soy estudiante - I'm a student
Soy doctor - I'm a doctor
Soy rubio - I'm blond
These have varying degrees of permanancy, from not very permanent to fairly permanent. But they all have something in common. They are a definitive trait/characteristic about the person.
I think it's a mistake for language teachers to bring up this idea that ser refers to permanent stuff and estar to mutable stuff. If you look closer, that's not the case at all.