Is brain retraining applicable when you denied your illness for 8+ years?

I have the illness now about 9 years. During the first 8 years, I definitely noticed a whole bunch of symptoms, ranging from the cortisol/adrenaline surges, to the crashes, PEM, and crushing fatigue. I did often google symptoms during that time, so there was worry, but I didn't know about ME/CFS yet.

During that time, I wanted to out-train my illness and push through. Because I was constantly active, I didn't really notice the connection between exertion and PEM. It was just that a lot of the time, I felt like complete shit but I went on, driven by cortisol surges and discipline. My VO2max was in the ''excellent'' range for years while my actual capacity was far smaller than before I got ill, and while I was also running on cortisol, adrenaline and discipline. I was very much ill, but unsure what it was, and thought for the longest time that I should just ''force'' energy by being in excellent shape.

This didn't prevent me from having symptoms at all (worsened them! On numerous axes) and being unable to work full time as well as a bunch of other limitations.

I did improve a lot when I found out about ME/CFS and started doing nervous system work, focusing on actually allowing my body to rest (instead of pushing through), focusing on physiological (perception of) safety, and using the polyvagal theory as well as a bunch of other stuff.

A medical event caused an unprecedented crash this year, very bad, truly indescribable. I'm slowly crawling out of that hole.

As such, I'm also interested in trying brain retraining. But, my question here is: is brain retraining applicable to my situation if for many years, I had zero anxiety about exercise, didn't anticipate PEM, didn't ''think myself ill'' (besides during bad periods/flares)?

Any thoughts, experiences, observations?