I know a few things that can be helpful, though it's still a hard row to hoe. But saying I know anything useful often just gets me called crazy by internet strangers -- just more evidence that our own personal hells are all too often compounded by other people.
I hope you can make your peace.
This is not true. "Mental health issues" may be a broad field. I guess US prisons are full of such people, some beyond repair. But I know people who got hospitalized because of health issues and they are doing quite well today.
There is help. There are medications and things can normalize.
I have a friend who developed mental issues and, as far as I know, is roaming homeless in Canada. But this must not be the rule.
They can, but the odds and process aren't the greatest. It's a pretty common experience that people spend years going through different treatment options before they find something that really works. For major depressive disorder, something like 15-20% of patients never find something that works consistently.