Like I’m still in monochrome but I still have things in my life with color to keep me going. I’m still trying to figure out if it’s the toll my meds took on my brain or if it’s just adulthood. Maybe both.

Probably more a mixture of ADHD and adulthood, honestly. I totally relate to the monochrome with bits of color. I've felt like that for the better part of a decade and just always assumed it had something to do with the ever present anxiety/depression that ride with the ADHD. I never took Ritalin, but I was on adderall for a few years in college after I finally got diagnosed; didn't really have any mental fatigue symptoms like you described, just made my body constantly run hot.