Living alone teaches you that “messy” was never your personality. Somebody else was just always cleaning it up.

Traits often turn out to be arrangements. While somebody else was absorbing the tidying, your mess never reached the point of consequence, so you learned to describe yourself as a messy person rather than as a person whose mess was being handled. Living alone removes the buffer, and the trait quietly changes.

You do not have to move out to test this. Take full responsibility for one space for a fortnight, your room or the kitchen after you use it, and see whether the personality holds. Most of these labels loosen under slight pressure.