I genuinely had a medical problem that screwed my 3 and a half year at high school, but I’m afraid adcoms will see it as “trying to make excuses for EVERYTHING” and backfire.
If it severely impacted your grades and there was nothing you could have done to avoid it and you think you can convey information regarding it that would help your odds of admission, then I guess write it.
Can you ask your GC to talk about it in their recommendation?
This is something your GC should be able to address in their recommendation- too often, if a student tries to write about it, an admissions officer might take it as trying to make excuses (even if it isn’t).
Agree it would be much better coming from your GC in his/her recommendation for the following reasons: 1) it is more believable coming from a third party and 2) it won’t sound like you are whining/complaining.