Why does 'to mentalize' not exist or what word should I use instead?

The word mentalize does not seem to exist. The closest thing I found was https://www.thefreedictionary.com/internalize. The 2nd description of internalize states

To take in and make an integral part of one’s attitudes or beliefs

This seemed to be used to describe what I am looking for to describe, sort of, but not close enough in my opinion. In the literal sense of the word internalize one can safely assume that something is taken from external to internal. The does not imply necessarily that it is about a human (or any other sentient being). Hence the first description of the word internalize is most pure in my opinion:

To make internal or cause to become internal.

What I am looking for is a word that describes the functional part of internalize, and explicitely targets the mental aspect of a being. I expected therefore that ‘to mentalize’ would exist, but appears not to.

Whereas mental means (2nd description):

Executed or performed by the mind; existing in the mind: mental calculation; a mental image..

So to make something exist in the mind, where it did not before, like an image or a document with specifications, I would say ‘to mentalize’.

When I Google for an alternative I get the following suggestion:

Understand and comprehend also do not match close enough for what I want to describe. As they are both about knowing, understanding, becoming aware of, to learn. But what I want to describe is make something exist in the mind. It could be so that I have something in mind, like specifications in a document about how something should be build, that don’t make sense to me. Hence I do not understand, yet I have it mentally in my mind. Perhaps the specifications don’t make sense, and thus understanding them in a functional way is impossible.

So why does the word ‘to mentalize’ not exist or what word should I be using instead?