It's very difficult.
I have a severely disabled dd who has many health conditions and I have 4 other children and always have that guilt feeling like all parents /carers of disabled children.
I'm in a different country than you but I do understand the lack of services and help available. People not in the know think it's as easy as picking up the phone and making an appointment to get help. I'm sure you've done everything, that's what we do, fight.... I'm 4 years waiting on respite, we're only looking for one night, it's to get dd use to staying overnight in the unit before we need it more, as I have a fatal disease and I don't have long to live so I need to know before I get bad and die that this is in place.
Is your ds on any meds? The reason I ask is because dd is on alot of meds, she has 2 types of epilepsy and her combo changed as she was having more seizures and it turned out the new medication did not agree with her and caused aggressive behaviour, we had to wean her off it and onto another one which took nearly a year so we had a full year of bad behaviour. It wasn't her, it was the medications. We find this happens when a medication is increased as she grows or one changes or is added.
The thing we try to do or at least did when all the kids were younger was work days out around what she liked, she would nod off in her wheelchair and that's when we'd go for food or do something the other kids liked. We're different as dd likes being out so I don't have it as bad as you where your ds doesn't like it.
It's difficult, it's tiring and people will always try offering advice but sometimes all we just want is to vent without feeling judged. You aren't failing your other kids, you are all adapting to a different life than what you planned or imagined and while it feels we are neglecting them it's usually the other side of the coin, and we over compensate trying to make it up to them.
If you have a liason nurse maybe you could have a chat and tell it how it is, we do tend to smile and say we're fine but if this is a possiblity it could be the start of the ball rolling into getting you help.