I am stuck at a point where I want to start opening up another side of the character's mental state but am having a hard time deciding how to pace it so it is a slow building rather than a sudden change in mood or if I want to keep the sudden change in mood as a sign of his illness. Also have problems because I am drawing from my own experiences and worry about how much of myself to put into the novel.
Before you devote time to writing it with the possibility of being disappointed with the result, I would sit down and write out a plan for this part of the story, just a breakdown of how you want to establish this other side of the character's mental state. This way you can tell how it develops over several thousand words. And if it's too slow or too fast, you can tweak the outline without having to rewrite and rearrange the story.
Also have problems because I am drawing from my own experiences and worry about how much of myself to put into the novel.
This depends on the writer. For me there is little separation between the novel and the author. Regardless of the character or the premise, the act of writing a complex and unique narrative experience is inherently an autobiographical expression. It begins with honesty--understanding the purpose, the setting, the characters, the plot, and yourself in honest terms, casting them through the lens of style and genre and point of view and inventing a representation of the author that while possibly not resembling them at all is still incontrovertibly theirs and them. One creates great art by not hesitating or fearing exposing themselves; it's vulnerable, and it is revelatory, complex by nature rather than design, intricate by way of one's humanity and simple in its appeal to other's humanity.