I used to do "proper" mindfulness sitting practice in a Buddhist tradition (I'm no longer a Buddhist). What I found striking about that kind of practice was that it doesn't stop the internal chatter; it's not supposed to. It renders it visible (and the speed of the stream of thoughts is startling).

Commercial "mindfulness" training is sold as a way of calming the mind. I think it's unwise to approach mindfulness with that expectation. It can have the opposite effect; things can get stirred up, especially if you do longish sessions. Commercial mindfulness is also pitched as being completely safe for everyone. It's not, it can cause trouble, and it's advisable to have a good instructor on hand.