It's a good question, Tides.

I'm not sure if you're referring to talking about a particular situation or feeling, or if you mean that talking about ANYTHING no longer seems to help. I'll address the former first:

For me, when talking about a situation or feeling no longer helps, it's usually because I've gotten bored listening to myself rehash the same thing over and over. I almost look at it as a wake-up call to myself -- if it stops being helpful to talk, then it's probably time for me to figure out how to move on.

However, if you're referring to the concept of talking about ANYTHING no longer being helpful, then that to me sounds exactly like what depression can do. Apathy can be very dangerous. My husband, when he was severely depressed, really clammed up and when urged to open up, would say "what's the point? nobody can help me. nothing will ever get better". It was awful. If you experience that or have observed someone else who feels that way, I'd say that it's definitely time to see a doctor if you haven't already.

Welcome, by the way...