Important to remember they don't care what you feel. Its a exercise of small talk. Goal is get them to shut up asap and move on with day politely.

Personally, I would struggle a lot too with people wanting me to act fake happy so I understand.

Don't fake it. Life's too damn short.

Optimize the decision pathway to maximize your happiness while retaining the job till its no longer necessary. That can be in multiple ways.

Outside the practical mechanics of the interaction, you should be allowed to say fine cuz who gives a fuck honestly.