When you accustom your mind to sensory overload, then even the basic thinking process feels like over-thinking to you, although, all the while you are just Under-thinking.
I have seen many under-thinkers calling themselves over-thinkers just because they are used to impose their belief on one fact over and over again and thus, jumping to conclusions without any proof to validate them whereas they miss all the other possibilities about the same situation or fact, that even any normal thinker can think of.
In fact, some people just want the title of an over-thinker because it makes them look or sound smart or smarter than others.
In most of the cases, those who call themselves over-thinkers are just Over-Feelers and Under-Thinkers.
Over-thinking is analysing every aspect of anything for which you need to first think of all the possibilities related to it and not just impose your beliefs on only one aspect that you or your mind could grasp immediately.
In summation, if you can’t think of more possibilities, over one fact or situation than a normal person, You Are Not An Over-Thinker, rather you are an Under-Thinker.