You aren’t selfish.

Having problems does not give someone automatic precedence over everything else that is going on in the world.

“I can’t discuss that with you right now.” is completely appropriate I think.

“I just can’t discuss that with you.”

“I’m overwhelmed right now.”

“I’m sorry, I’m not the person for you.”

These are just some ideas. Repeat civilly and firmly as often as needed.

Get away from the isolation with the person promptly and firmly.

Firmness is key.