Learn English – What’s the adjective for someone who isn’t loose with their thoughts/opinions/judgements

In my professional and personal life I've been trying to listen more and talk less. I tend to offer my thoughts/opinions/judgments where they're not needed and I'd like to stop doing that. It's been bugging me to find a name for a person who isn't so loose with their thoughts/opinions/judgements. The name for a person one might say "Oh yeah, David, he's a very ______ kind of person." Someone beyond reproach keeping gossip and such to themselves…