When should you start using a service mesh, anyway?

We've been breaking down our big application into smaller microservices for about a year now. Right now, the services talk to each other directly using REST calls, and we have some basic retry stuff in place. But as we add more and more services, handling all these direct conversations and making sure they're reliable, you know, dealing with timeouts, circuit breakers, and security between them, is getting pretty annoying. I'm wondering when you all decided it was time to bring in a service mesh like Istio or Linkerd. What problems were you facing that made you do it, and how did it turn out?