my ds, now 20 and in 2nd yr at uni, was (is) very similar. Also undiagnosed autistic.
he still, and will always I suspect, tend towards the negative at any given time. No real friends at school, bullied for a while, didn’t leave the house between 14-16 etc.
With regards to being grateful for loving parents, nice house/food/holidays etc he believes that that should be a norm, not exception, so nothing to be grateful for per se.
he’s now 20, physically gorgeous and at the highest rated uni in the uK, has real friends who love him, a beautiful understanding girlfriend, and is fantastically lucky, and yet can still act as though the weight of the world is on his shoulders.
however, and to give you hope, he can now put his opinions in marginally more perspective than he would have done a couple of years ago. He can (almost) laugh at himself, and is beginning to check himself when he realises he’s being unreasonable.
With more exposure to the rest of the world he can reflect on his circumstances, and he does so positively.
give it (more) time. The only thing I personally did which helped was not to feed the wrong wolf - if you understand the reference? Briefly sympathise if you seriously think a situation warrants it, otherwise ignore, walk away, or move on to a less contentious subject.
Show real support and interest in any attempt at positivity, and praise (even if it feels ridiculous) enthusiasm in anything - slowly my ds is learning that he gets so much more attention (which is what we all crave after all) when he’s demonstrating an ability to be more balanced.
Some of them are a work in progress until we die I think.