You can answer honestly but it’s a greeting, not an invitation to take over the whole conversation to talk about yourself.

Them: “How are you doing”

You: “Pretty bad to be honest, but I’m hanging in there”

And from there you either get:

Them: “I’m sorry to hear that I hope it gets better”

You: “Thanks”

Them: “So I wanted to ask about your TPS reports…”

Or :

Them: “Oh no, what’s going on?”

You: “Well I’m having a lot of mental health issues…”