What exactly is disillusionment?

There is a difference.

When someone is disillusioned, they have the feeling that their previous perspective was an illusion, and now they feel they perceive the disheartening truth.

Disappointment is generally a broader, probably less significant, perhaps more temporary upset.

I was disappointed that the book I was looking forward to didn’t cover a topic I hoped it would. But I was disillusioned when I read chapter 5 and realized the author I’d thought I liked, is actually someone I can’t respect.

I think most people tend to see being disillusioned as a negative thing. I disagree.

To me, being disillusioned means having all illusions stripped away and looking squarely at reality in all its flaws. It may feel traumatic but I think it’s a very good thing. If you can’t deal with reality, you’re going to continue to live in an imaginary world. that’s not a good place to be.