I once spoke to someone who put the stress of chronic symptoms (and their recurrence) as this: picture your brain as a bucket. Every day, drip by drip your brain (and your mind) fills with stress. Everyone's bucket also has a hole that can open up at will. Notice I say "AT WILL" this is very important. Many people's bucket fills, but it also opens drip by drip out the bottom. The bottom drip isn't necessarily regular, and it doesn't have to be. As long as that bucket stays below the maximum fill line, and occasionally (or even regularly) that bottom opens to dribble out some drops, you have maintained some sort of homeostasis. But when that stress meets your maximum fill line or even goes a bit above it, we usually begin to get some sort of signal. It doesn't have to start with chronic pain, but our bodies send us messages and we often have thoughts, even fleeting that something is a bit off. It's our attunement to these messages of brain or body that helps us reel things back and get some perspective. However I think most of us try and sweep these feelings under our internal mat. That might have a lot to do with our personality traits: we don't want to admit these messages are there because we can handle things "perfectly" or we tend to attend to anything or everyone else outside of ourselves instead of to our own needs. Things sneak up on us.
I absolutely believe it's that combination of things that
@BloodMoon suggests, and then heaped onto that, is
@Adam Coloretti (coach) 's wise observation.
Our internal dialogue, our ability to choose to react or respond to situations, and our internal observations and also utilizing the skills we learned from Dr. Sarno put into regular practice.
Many people are devotees of Sarno, but I think we only know him from is books and that might be a limiting aspect of his actual "prescription" to real-life clients. Sarno's ultimate "commandment"

is to get back to normal life. However, many of us with TMS were not taught or didn't learn some of the skills that help with that "normal life" to maintain a better equillibrium and internal balance. We let that bucket reach it's maximum fill line without even noticing. Sarno teaches us (some) of the skills to open the bottom cap and let the stress dribble out - eg. journaling, but we now know there are other methods that help, and I think many of his patients who had recurrent intermittent symtoms (many of whom reported they would call him, get a pep talk and resume their "prescriptions" and feel better) learned to keep empoloying the stratagies he taught them through their life. Eg. journaling simply became incorporated into their normal life - once a week, every few days, when they could observe or feel a stress build up. They kept honing their internal awareness to KNOW when the stress was building up, they learned some stress management by advocating for their needs and meeting those needs regularly Eg. quiet, solitary walks in nature, meditation, taking breaks during the day and asking themselves how they feel emotionally, and all kinds of other things that help open the cap of their stress bucket.
It's not a perfect method to never feel another build up of stress or have more chronic symptoms - that was never promised. Sarno teaches us a way to deal with it, with more confidence through knowledge of how to deal with an episode when it happens.