It's always a dopamine regulation issue, always. It's not a person in particular, it's just that you've objectified an idea about this person and are looping because of the autism.

Hm, okay. I'm nearly 10 years your senior and I still do this. You just have to sublimate and redirect the energy somewhere else. Actually, it's fine to obsess but just don't make any of it public and don't interact directly with your object at all.

You can wear yourself out by writing about it, or doing some sort of other creative activity.

I would recommend disconnecting from the Internet entirely, and when you feel yourself getting dark, to just stop communicating with them until you're level headed again.

Autistic obsessions can be controlled, but just realize it's almost a form of OCD. Don't freak the other person out by saying what you feel (god help you if you have impulse control problems), but just... meditate.

I can't recommend it enough. Disconnect from the internet, walk around outside and meditate. Usually obsessions are escape tools from stress, discomfort, anxiety or pain.