How do I explain my mental illness to my husband?

Look at Amazon books I'm bipolar and found the Bipolar Survival Guide and it's actually written for the friends and family to understand. If you don't have bipolar disorder I'm sure you can find something there closer

Maybe go through a mental day to day with him. Like what your brain thinks and feels while doing basic daily stuff.

I showed my husband youtube videos, of people going through the same stuff as me, that were able to explain my difficulties in better words. Mental illness is very abstract, even for the people suffering it, so it can be very hard to describe.

Quite literally break it down, start with the way you're feeling, and give simple explanations to the causes