I'd like to join Sophie_M in welcoming you here and greatly sympathize wit your circumstances. Reading your words it seems to me you are really reaching the end of your strength and have spent it all on your son, understandable, that's what mums do.
No matter what you assume you are not an inexhaustible well of strenght you can draw on forever, you are human, though probably stronger than most.
This is a sign overhead in passenger aircraft (yes, there are sill some flying) which says:
"Put oxygen mask on self before helping others".
Which when you think about it makes sense. If you reach the stage you pass out you are not going to help anyone.
Why do I say all this? Because you need support, and you need regular times in your life that are for you to relax and enjoy, not be in ceaseless worry.
So first may I ask you if you have medical support? You mentioned depression - are you being treated for it? Even if you are I'd strongly suggest you see your doctor, explain you are living with the fear you son might kill himself, or at least is permanently very unhappy. See where that takes you. Hopefully to counseling, therapy and effective treatment.
Do you have anyone to give you a hand, someone you can talk frankly to about things, and they will care about you? Not being alone does help.
I'd also suggest you have a habit you enjoy of an evening, something you can look forward to each day and will take you out of yourself. It does not have to be special, I use books, what might you like?
With your son, life has given him a lot of very hard knocks, on the spectrum, no luck with employment, and his friend's suicide. That's more than enough to make he lose confidence and feel hopeless. Sadly the young lady did not help, raising his hopes then dashing them.
For him to I'd suggest medical support, though I expect you are already on top of that, however if you are worried he might talk his life tell his medical team. Nobody keeps another alive all by themselves no matter how much they love and try. It realy is a team effort. Sure those that love play their part, but so does the medical people and importantly so does the person themselves, even if only a little bit.
There is more I want to say, you have been shouldering a huge burden, and I'd like to make it lighter -no instant fixes, but over time. Basically your son's confidence needs to be restored to what it once was.
I do hope you come back and we can talk some more.