I think Genetic has great points and if you can slowly rephrase your message, removing the mental illness component to the extent possible and substituting it with your realization of the importance and the value of the friendship and how you want to be a good friend in the future - something along the lines.

You have no obligations with respect to disclosing your medical history. You just want to make amends and improve the friendship, right?

So you should not feel obligated.