Thank you Paul.
For those game enough to attempt the "mind watching itself" practice, please note that the only way to succeed is to catch thoughts as they arise, BEFORE becoming entangled in them. It will then be noticed that the simple act of noticing a thought, feeling, emotion as it pops up makes it temporarily vanish (be sure it will come back !). Emotions cannot cohabit with the watcher. At first, it is best practiced with commonplace, everyday thoughts, not the obsessive kind.
I used to turn practice into a game played with my restless, over-reactive mind...trying to pip it at the gate of awareness. Having fun with it somewhat balances the frustration of consistently being beaten to it. As we become more proficient, there is watching the space between thoughts and widening it, abiding in it. A skill like any other just particularly difficult to master.
The trouble with sitting meditation or intently observing one particular object is that the mind gets bored...so it automatically becomes distracted. Walking meditation is easier. For example going for a walk with all senses on the alert for everything we come by. Focusing on what is to be seen, touched, heard, smelled or tasted instead of being in our head with senses on automatic pilot. Whenever the mind takes over, just notice, acknowledge and return to thought free observation. Overthinking is a self-perpetuating activity. If less attention is given to it, it eventually calms down. Eventually.
It is amazing how much of Life passes us by unnoticed while we are ruminating past experiences, concocting unlikely scenarios, replaying conversations and generally torturing ourselves with a future that will probably never happen...at least not the way we imagined it would. What we must do is move on to a different object, person, situation before becoming emotionally involved with it (whether we like it or not, what it reminds us of etc...).
Much of the exhaustion felt in depression/anxiety is due to the stress of overthinking. Being fully present in the moment frees up an incredible amount of otherwise misdirected energy.
Besides, considering the mind's insistence on playing tricks on us, turning the tables on it is only fair :-)
The mind watching itself is the beginning of control and a huge step towards knowing ourselves.