How do I prove that he owed me a fiduciary duty?

I am trying to find proof that my husband owed me a fiduciary duty when he drafted our Separation Agreement and had me sign it. Even though I was suffering from a recent traumatic brain injury (for which I have been deemed by the court appointed evaluator as disabled) and asked him for a guardian, and said I didn’t understand the separation agreement. I tried to hire a lawyer to explain it to me, but she refused to represent me because she had serious concerns about if I was well enough to make major decisions. Later, I found out my husband put untrue things in the separation agreement that benefit him financially at my expense. Plus he put in very low child support, $25 monthly for each child, and he only paid it a few months.