nc here too.
I'm so sorry that you're feeling this low OP. I know I've thought seriously about suicide when dealing with DS's SN and I'm sure it's not uncommon.
I have to say I'm not sure how much Samaritans really helped me - I wanted practical help, I was sick of the lack of hope for DS and me and having to deal with so much on my own (single mum). So having someone at the end of a phone line listening but not able to actually do much, wasn't a massive help. 999 is actually more useful from a practical pov because it has to go on your records, which affects how agencies deal with you.
DS is in residential school now - more because of his own needs which can't be met locally, but it's made a massive difference to both of us. DS finally gets all the provision he needs, without additional fighting, and the school is a specialist so I'm not constantly having to monitor provision and trying to explain his needs to inexperienced staff. Residential school is absolutely not a failure - in fact he's doing much better there than if I'd kept my head down and insisted he continue in local provision.
Definitely go to your GP - I didn't realise it at the time, but it's a very useful trigger for getting action on your case from SS, LEA etc. Not always in the most sympathetic way (it's very, very intrusive) but it will get you practical support in situations that might otherwise get dismissed. I took ADs but it takes a while for them to kick in. I think they're useful for keeping you going, though - and there are many different types now, so if one doesn't work well for you, keep asking for a different type.
I've heard some parents say on here that they don't want evidence of MH issues appearing on their records in case it's used against them, but I think in severe cases like this, it's actually more useful to have it on your records.