[non autistic OP] My partner is in a doomerism spiral because of their autism and I don't know what to do

Hello! I'm very sorry if I get anything wrong and I am here to try to learn and understand

I (M21) have been in a relationship with my partner (NB21) for over 2,5 years. I don't know if they have depression, never confirmed, but they definitely have autism and ADHD, recently diagnosed. Their main issue as the themself put it is that they refuse to do Literally Anything Ever. They're 21 and the only thing they ever do is consume content about their hyperfixatiins which they also hate and say that all the "cool autistic queers" are fixated on "cool things" like drawing and fandoms and not "boring stuff" (their special interests are cartography geography ans politics which I think is really cool but they dont agree and force themself though tears to get into fandoms they dont gaf about). They do university work once in a blue moon and they have meltdowns over the pressure, hitting themself and telling and crying and it's horrible. but they refuse to do anything to accomodate themself in any way and it leads to them just bedrotting. I feel like I can't understand them and they can't understand me and it's driving me insane. I beg them to at least do one simple task together. Nope. They just shut down and tell me in a meek voice that they can't/will die soon anyway/their life is over because they're autistic/etc. They are constantly negative about every little thing, say I will never understand because I'm allistic and I ask them to explain and they refuse because they say it's all overwhelming and pointless, they're "an r-slur anyway and it doesn't matter at this point". I offer to do anything for them. Nope. I offer to parallel play. No. I offer help with homework. Nope. I beg them to get therapy. Nope — tried it and it didn't work so clearly therapy sucks and they're never doing it again. I research autistic adults support groups in our area — No I don't wanna and also stop parenting me. Then it just turns into either them being all meek and sorrowful saying they will do whatever I say and how much better I am at everything which makes me want to bang my head against the wall, or they start saying something like oh so I'm so difficult to be with aren't I? And it makes me bang my head against the wall too. I'm so lost I feel like nothing I do ever helps I can't watch them do this to themself but they refuse to even try, they're neck deep in their doomerism and refuse to accept any help whatsoever, my heart breaks I feel so hopeless and helpless and also I fear that I'm starting to grow resentful.

They have a trust fund so money/work isn't a problem for them right now thank God.

What is going on? It's been like this for the entirety of our relationship but they've been diagnosed with autism only recently and I feel like instead of getting them help it just made them more hopeless. Please please help.