It does seem impossible and utter madness. And I am sorry you have lost your husband to this horrible illness. Our loss is different. You can try to wrap your head around suicide from a thousand different perspectives. It will Never make sense. I'm not sure how far you are out but I felt some relief from the anger, depression, and guilt after I started thinking of my dh suicide as a result from a brain illness. An illness, like undetected heart disease that has had unfortunate and dire consequences. I Know without a doubt how much my dh loved his children, there is nothing in this world that meant more to him. So when I think that whatever happened in his brain the night he died, I know he wasn't in his right mind, wasn't thinking of them or me.
I didn't know, you didn't know and even if you knew things we rent quite right for your husband, in all likelihood there is nothing you could have done to stop it. No one person is all that powerful against such a powerful illness. Take a deep breath, and try to go easy on yourself.
Erin