Yes, I can't believe my H did it either. He was a total hyperchrondriac! Always worried about his health.
He wasn't depressed, classically, I don't think. But I knew he was very low, panicked, paranoid and irrational. I had thrown him out a week prior upon discovering an affair and refused to see him. His high status job had gone as his affair was at our shared workplace and he had been very senior, as am I. He blamed me utterly for this in his last day, even though it was his own drunken flagrancy that had alerted the boss to what was going on (although he was right that I wasn't fighting for him to keep his powerful job, I had left it up to the firm and even asked them not to fire him outright but just move him to another role somewhere else in London).
This was after a year of EA and refusal to leave plus neglect of our kids. After his death it emerged he was seeing at least one other girl at the same time. I think he possibly was mentally ill but in the sense of being a narcissist, rather than in a benign way, IYSWIM. Still, he had been a good man only two years ago and for the eight years of being together before that. I don't know if narcissism-style illness can come on just like that. There is a good chance he just had an affair, followed the classic behaviour there, and then just did it following the collapse of his job. Who knows. He had cultivated a bit of a 'victim' persona in his last year or so and so I was shocked beyond belief that he did it (you can't get sympathy when you're dead). I would have expected maybe a cry for help, attention-seeking 'bid' to make everyone feel sorry for him but not the sure fire method of tube jumping!
Anyway, not to hijack. Sounds like your H was a good man who felt depressed. Rather than someone who wanted to escape from their own self-made problems. Reading up on it afterwards, most suicides are men who were 'quietly depressed.' It sounds like the note has provided a bit of comfort to you (mine didn't but it was obvious 'why' I guess).
I told the children he was 'ill in his mind' and so made that decision because he felt too sad to carry on and now he won't ever come back. They accepted it. How old is your DD?