From what you've said my guess is that your eldest is probably the biggest drain on your emotional resources.

Have you joined a support group for families with special needs kids?

There's no substitute for interactions with other folk in a similar position to yours.

Disclosure: Two difficult kids, no outside support while they were growing up. Seven years ago, around the time our eldest moved out, our youngest had their first hospitalization for a psychotic episode and is now formally diagnosed as schizophrenic. Loving them ain't ever been easy and it's meant giving up on most of our personal ambitions but I wouldn't want either of them out of my life even if you put a gun to my head.