In my fantasy novel-in-progress, one of the two main characters has a close friend he's known since childhood. Both of them are greatly developed by their interactions with each other throughout the story, and this character is critical to the story as a whole.
The story takes place in a very restrictive Victorian-esque setting, the character's full backstory is only revealed much later as part of a major plot sequence, and he presents himself as masculine (by which I mean that he dresses in traditionally masculine clothing and refers to himself as male). For all of those reasons, it is not immediately obvious to the reader that this childhood friend is a transgender man, and the character himself does not explicitly spell it out until a conversation with the main character a few chapters in.
In other words, this character being transgender is only one part of his character, and thus far I have not focused too much on this aspect of him; however, lately I have begun thinking about developing him more and giving him more scenes and dialogue, and that has led me to this question...
As part of developing this character, I have been thinking about how to accurately portray the thoughts and internal struggles of a character who has gender dysphoria in a way that is accurate to people who have gender dysphoria in the real world. Specifically, as part of a scene later on, I would like to write a brief POV scene of a traumatic moment from this character's childhood that involves his feelings being belittled and challenged by his non-accepting mother.
However, I do not have firsthand experiences with the thought processes involved in gender dysphoria and how a person would mentally and psychologically experience it.
How do I portray the thoughts and feelings of a character with gender dysphoria in an accurate, sensitive and positive way?
Edit: Additional information
I would like to clarify that the reveal is not sudden, unexpected or "out of the blue" - it is made fairly clear when the character is introduced. When I say that it is not explicitly stated until later, I mean that the character does not make an outright reference to his status in dialogue until a few chapters in, since the main character has known him for a long time and I didn't want them to exposit their backstories at each other in their first conversation solely for the reader's benefit.
I would also like to clarify that I use gender dysphoria as a general term for a person who feels that their gender identity does not match their physical characteristics, and do not in any way suggest it is a pathology or disease. That would be horrible, obviously.