Ego Collapse
The ego is defined by psychologists as "the part of the mind that mediates between the conscious and the unconscious and is responsible for reality testing and a sense of personal identity." It has two major components important in tennis; Fear and Conceit. Fear is of the horror of embarrassing oneself: double-faulting at match point, missing the easy, sitter, shoulder-high volley, or just tripping over your own feet (my personal favorite). Fear reflects the ego taking full responsibility for the outcome of every point. That fear can be paralyzing, as we shall see.
Conceit
The curse of conceit is a touch more complex. Why is it that we can hit three great shots to set up an easy winner and then blow it? Sometimes it is fear of "missing the easy one," but there are times when we are feeling great about ourselves and still slap that overhead over the baseline. Often we are surprised and perplexed at ourselves: We know how to hit the shot - we thought we owned it - so by what dark magic do we miss it? The sequence of subconscious events goes something like this: We are in the zone, and a point comes along where we find a nice groove. Our thalamus has taken over, and a glorious feeling it is! We hit an intricate series of great, forcing shots, and our opponent coughs up a big, juicy, shoulder-high sitter for us. We decide to show off and give that sucker a ride, so we give it one - right into the back fence.
The problem here is that the ego has stepped in and has passed control of your motor nerves from your thalamus, where you store your well-practiced strokes, to the cortex. The cortex is the seat of the ego, which fully understands and appreciates the opportunity for the glory that this sitter affords and would very much like to bask in the glow of the spectacular winner that is about to be struck. That's fine, but the cortex would not know how to hit a high volley if its existence depended on it. The cortex is a "NO!" engine. It communicates to other parts of the brain through inhibitory neurons that shut things down. It is designed to prevent you from putting your hand in the pretty campfire, not to initiate a deft sidestep when someone throws a pie at your face.
The thalamus, if allowed to do its job without interference from the cortex, would execute a well placed, medium-paced volley which would, in all likelihood, win the point. But that just isn't good enough for your ego, is it?! You want to prove how strong you are, so instead of hitting a crisp, sliced high volley, you slap at the ball like a rank beginner and then howl as it beelines beyond the baseline. Nice going cortex! Your cerebral cortex is not qualified to take over a tennis match. That isn't its forte. But the ego lives in the cortex, and the ego wants ownership of this exciting contest, especially when you get dished up the sitter of the year. One thing the ego is good at is taking credit for things - like winning a tennis match. It irks the ego to have to relinquish control, and with it, the thrill of victory, to the dumb old thalamus. Allowing one's ego to take over-represents the sin of conceit.
Fear
The mechanism by which the ego's fear disrupts your game is similar. Let's say your forehand has not been up to snuff so far today (or ever?), and your opponent hits a nice easy shot to your forehand. Based on recent history, your ego is terrified that your forehand is bad or broken or something and decides not to let your thalamus execute your normal stroke - the one you own and have hit ten thousand times. Instead, it paralyzes the thalamus and passes control over to the all-knowing cerebral cortex, tasking it to invent a "new and better" forehand on the spot. The cortex has 30 milliseconds to do so. The cortex, which isn't all that creative under pressure, responds with the only word it knows; "No!" At this point, the ego panics and hands the reigns back to the thalamus which has been taking a bit of a nap and sees that it has 30 ms to execute a stroke that takes (with the preparation of the feet, trunk, shoulders, and forearm) 120 ms to perform adequately. The result is not pretty. Often the ego passes partial control of the hitting to the cortex - just for safety's sake, mind you. The result is random inhibition of elements of footwork, follow-through, preparation, spin, or a sort of general paralysis of the stroke that inevitably leads to a poor result.