I’ve heard it tons of times before and I am sure you have to.
“He’s actually a nice person” when describing someone who’s drunk out of his mind.
Or someone who’s having a bad day and starts to act like a jerk.
Or someone who’s really mean to customer service staff.
Or someone who lost his cool at the heat of the moment.
Or someone who is actually really an asshole, but his good friend insists he’s a nice person.
The problem here is that you being nice is a saving grace rather than a basic staple in your character. That is why others need to tell others for you that you’re actually nice, but you have trouble being it yourself.
It doesn’t take a genius to be an adult, so start acting like one.